Battery Parasitic draw test?

Hello, I wouldn't put your meter in series with the negative cable until you know how much of a draw you are having. You could burn your meter up if it's not fused. I'll explain the proper way to find a draw using the voltage drop method, but you need to prep the vehicle first.
Since you need access to all the fuse panels in the vehicle, but you still need the vehicle to go to sleep (if it will), all the door latches and hood latch need to be closed so when you lock the vehicle it will start to power off modules(go to sleep), then you can test for any voltage drop across fuses. I'll look up where all the fuse panels are, and what latches will need to be closed for it to power down. These vehicles have a ton of modules in them, so it can be any one of them staying awake or waking up a network causing a draw on the battery. You're going to set your multimeter on DC volts, but testing for very low voltage drop, in the mv (millivolt) range. This indicates current flow across a fuse.
You will need access to the trunk, engine compartment, and interior fuse panels. So you're going to have to make sure all of those latches are latched so the vehicle can lock, and then the modules will start to power down. It can take a while for all the modules to finally go to sleep. It would be best to use a low amp clamp around the negative battery cable to monitor current flow. That way if this a few amps draw it won't burn your multimeter out. How fast is the battery losing power?

Is this the CLK500 Cabriolet model?
Either way, you will start by latching the door, trunk, and hood latch (if it has one). But leave the doors and trunk lid open so you can access the fuse panels in those locations. Then you will need to lock the vehicle with a key fob if you have one. This way the vehicle will start to power down its modules. And eventually go to sleep. It can take an hour or more depending on the vehicle to go to sleep. When it has gone to sleep, that's when you start to check fuses for any mv voltage readings, if there is voltage drop on any fuse, then there is some current flowing on that fuse.

Note*
Pulling fuses to check for a draw is not the correct way to check, because once you plug that fuse back in it can wake up the vehicles network again and you have to start all over.

Also, if you notice any components that are not working correctly, i.e. window switch, radio, heated seats, HVAC stuff, etc., those are areas that might be causing a draw while the vehicle is off due to a malfunctioning component. Parasitic draws can be one of the most difficult issues to find, especially in a Mercedes or higher end vehicles because they have so many modules. This vehicle has 4 pages of modules.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

No, you don't tape the door latches, you need to push them in so that latch like they would when you normally close the doors. This way the module that monitors the door latch switches thinks they are closed and locked. I use a screwdriver and just push them in. They should unlatch when you pull the door handle, same as when you would go to open the door. You're just tricking the module so that you can lock the doors and set the vehicle into its power down mode. The modules in the vehicle won't power down if the doors or a trunk latch are open. If there are actual switches that the doors push on when they close, such as ones that may be in the doors' flame, then yes those need to be pushed in and held in by some means. I would think a Mercedes has switches in the door latches, but I don't work on them much.
You need all the interior lights to power down as well. Then lock the vehicle with a Fob if it has one, that should set the security system, and modules on the networks will start to power down as they normally would.
The tricky part about parasitic drains is that if there is a module that is staying awake, or waking up periodically, it can cause other modules to wake up.
But get the doors latched that you need open to access fuse panels, lock the vehicle, and give it an hour at least to start powering down. Don't leave any scan tools hooked up either, that will keep the network active as well.
I'll see if there is any specific time for Mercedes modules to power down. Having an amp clamp on a battery cable will tell you the draw, it can also help you to know when modules are powering down because current flow will go down.
The next thing you're going to check is voltage drop across fuses, set the meter to DC volts on a mv(millivolt) scale. And just go across each fuse. When everything is powered down you shouldn't read any voltage drop across a fuse. If you read 0.020v(for example), that's voltage drops across that fuse, and there is current flowing through that fuse. This eliminates pulling fuses. But you need an amp clamp on the battery to know what the current draw is when the vehicle is powered down. And in a case like this, where the vehicle has a ton of modules, I will also have an oscilloscope on the CAN bus network monitoring activity. There shouldn't be any. And I'm willing to bet there is module staying active on the network.
You can also use a thermal camera to look through the vehicle and find areas staying hot, that also helps to determine which areas might be drawing down the battery.

How fast is this battery draining? Is it overnight or a few days?

Watch this video, he goes over a basic parasitic draw test. This will give you a better idea of how this test works. This is very helpful. But its basic, not addressing module issues,
There are also a couple TSBs on this PASS system causing a battery drain due to it needing a software update. These types of issues are very difficult to diagnose because this is a subscription based wireless system, The PASS system will keep the CAN bus network awake because it's trying to update but it's not able to, and this can cause multiple modules to stay awake on a CAN bus type network.

https://www.youtube.com/watch?v=lRcj1fQcWwU

You can get a cheap amp clamp from Harbor Freight. If the battery is draining that fast, then you have something that is pulling a few amps at least. It's not just a small milliamp draw. That's why I was cautioning you about putting your multimeter in series with a battery cable. Most meters have a fuse, some don't, but you can only take an amperage reading for something like 15seconds and then let the meter sit for 15min before taking another reading. One of my multimeters has a 10amp max current check for a maximum of 15seconds, or it will burn the meter right out. So, an amp clamp will read the current flow by measuring the magnetic field around the wire. Thats why you just clamp it around either the positive or negative battery cable. Usually, the negative cable will be the one wire you can get a reading from because there's always multiple positive cables and you need to get a reading of the full current flow from the battery, Ill check Harbor Freights website and post one that looks easy to use. With a draw that large you're going to see it right away. You can set the voltmeter to 2volt scale, that just means it will read up to 2volts, if it's a few decimal points down, switch it to 200mv. You get the hang of it. The real purpose is just to see if there is any voltage drop. And yes, you'll need to lock the vehicle to get the modules and networks to shut down. But that might be the issue here. Let's find out how much of a draw you have first, then we can go from there. It might be something like an amp for the Radio that's staying on, those draw a lot of current flow.

It looks like Harbor Freights amp clamps are mostly high amperage AC clamps. You need one that is for lower DC systems. Accurate automotive amp clamps cost more because they need to be able to read lower amp settings. I have 2, one is an AESWAVE 20/65amp and the other is a Hantek 650Amp which is used for relative compression tests, but both of these hook up to my oscilloscope.
It looks like Harbor Freights amp clamps are mostly high amperage AC clamps. You need one that is for lower DC systems. Accurate automotive amp clamps cost more because they need to be able to read lower amp settings. I have 2, one is an AESWAVE 20/65amp and the other is a Hantek 650Amp which is used for relative compression tests, but both of these hook up to my oscilloscope.

You want something like this, easy to use, you can see here hes got a 5 amp draw, it has wide jaws to reach around any thick battery cables. It doesn't matter if you are on the positive or negative side. Current will flow. Just make sure whatever you get it says DC current clamp.

Okay, if you find a very high current draw, you can also check the temperatures of components, because something pulling high current flow will be warm and show up hot with a thermal camera or temperature meter. Do you know if the vehicle has an Amplifier that goes with the Radio? I'm seeing the option in the service info Sound Systems for the Convertible.

And are there any codes set in any modules?
These are the Radio with an Amp diagrams, there are 3 Wake Up wires as well as CAN bus network wires to the Radio, so if one of these is staying powered up, it can wake up the Amp and CD changer as well as these Antenna Amp modules. This would pull a lot of current flow.

Watch these 2 videos, they are of a Mercedes with a battery draw, its part 1 and 2, but just so you can get an idea of what I mean when mentioning modules going to sleep. And why we monitor the networks in some cases to see if modules are waking up other modules. This tech is really great, very smart guy.

https://www.youtube.com/watch?v=5OE3CYNSVNI Part 1
https://www.youtube.com/watch?v=3lUBPH9l3nA Part 2

The hood latch itself will not be the current draw, but it can keep a module awake on the data network. Gotta remember that the hood latch signals a module, and that signal eventually ends up signaling the Cluster to display that message. I would start there. These Front and Rear Sam modules act as Body Control Modules in these vehicles in some cases. But since you've already noticed the hood latch, thats where I would start. There are so many modules in this vehicle its amazing, even for a 2004. Service info only shows a mechanical latch for the hood, but if theres a message for it, there has to be a sensor/switch somewhere to trigger that warning lamp, And its definitely being broadcast on the CAN bus network if its a message being displayed on the Cluster and not just a warning lamp. Id have to search through the oem diagrams to find the hood switch and what module it signals, But Im not too familiar with these diagram types. Its probably the Front Sam, but Im not sure on that yet. There are 3 or 4 CAN bus network pages, Ill post them later for you to see.
See if you can find a data PID for the Hood latch state with a scan tool

I would unplug it and check for continuity when pressing it instead of jumping it out, Since we're not 100% sure on its operation, if it has continuity when pressed and then goes OL when open, the switch should be ok, you can check one of the wires for voltage with the key on, I don't even see any hood switch in service info, which is common. If you don't read any voltage on either of its wires you can try jumping it out at that point, But I think you'll find if it has continuity, the switch is not the issue, most likely one wire will go to ground, or grounds through a module to signal the hoods shut. If you read OL on the meter, you can try jumping the pins, but I'm hesitant on doing that without knowing its operation. You don't want to ground it if it's supposed to have some resistance to it, you might be shorting a module to ground. I'll look again for service info, but I didn't see anything before.

This is the hood switch I see in service info, is this what you have?

So far, all I've found with a label of S62 in the OEM diagrams is this latch switch here. Which was under the Grounds diagrams, maybe this U82 is the Module ID. The aftermarket diagrams do not show a hood latch at all, the latch is on a ground circuit, the trunk latch is not even called a latch.
It looks like the Hood switch is part of the Anti-Theft System and is on the CAN B network, so if that switch is not in its proper state, I'm sure it will keep that network awake.

You don't want any, if a fuse has any voltage drop at all there is current flowing, this is the chart that shows how much current is flowing according to the voltage drop across the fuse. Since fuses come in different sizes, the resistance for each size will be slightly different, and I mean very slight. So, for example, on a standard 10-amp red fuse, if you measure 0.1mv across that fuse, that means that 13amps of current is flowing on that fuse. This is an extreme example just to give you an idea, you will come across a fuse that is reading something more like 0.041 on that type of fuse which is about 5amps of current. This is all based on Ohms Law. I've added in another chart which deals in lower amounts of voltage drop. And a link to the PDF where the 2nd set of charts came from. But we will know more when you get your amp clamp and can get an idea of how much draw there actually is. I'm going to have another Technician chime in here, Steve, because he works on Mercedes and will be able to give you some better direction as to what might be some things to check right now. I'm just showing you the best way of finding a draw without having to pull fuses, because that method is only going to give you skewed results and could send you in the wrong direction.

https://caravanchronicles.files.wordpress.com/2018/10/fuse-voltage-drop-table.pdf