Battery Parasitic draw test?

52amps to 23amps? A half amp draw is high, but I expected it to be much higher. How many times has the battery been charged and fully discharged?

The aux battery might be discharged as well though, but really the current draw on these vehicles goes down to about 0.016A(16ma) when the entire vehicle goes to sleep. Thats what I have seen before, but you want it to def be down below 0.050A. 50ma is the max I think a vehicle should be pulling after power down. And if you see it fluctuating that might be other modules waking up because of activity on the CAN bus, Most CAN bus modules send out a data packet or type or ping letting other modules know they are on the network, and when other modules don't see that message, they will set a U1000 code for the module they are not seeing, It's kind of a telling on each other code. But that amperage has gone down. The Aux battery is going to draw off of the main battery too when the main is hooked up after being charged, so something to keep in mind too. I know those aux batteries are pretty small, but they will still try to equal out with the main one as much as possible. I think you may find that there is more than one fuse that has a voltage drop, because of multiple modules being awake. This is why I'd be monitoring the CAN bus signal as well. With a draw like this, in some cases you will see part of the network go to 0-volts, but then another module sends out a message and other modules become active on the network again after having gone to sleep. These are some of the most difficult diags to do.

You might want to watch these two-part videos again, since this is a Mercedes with the same CAN bus multi connectors.

https://www.youtube.com/watch?v=5OE3CYNSVNI

What's the charging system voltage when it's running? You can also measure the amperage coming out of the alternator on the main cable with an amp clamp.

Something else you should invest in is a thermal camera, it can show you any modules that are potential current draws real fast, when radios or clusters are staying powered up, they show up glowing bright, it can really help with finding a draw or at least narrowing it down much faster than going through all the regular testing methods. Just something to think about, I know you go through a lot of vehicles. Even a small one like this will be a huge help. Start watching some of these YouTube videos as well. This tech, Jake, the one who is doing those 2 Mercedes videos has a ton of great videos if you look on his YouTube page. He is super smart and deals with these type vehicles all the time.
This is his link:
https://www.youtube.com/@autodiagyt/videos

You can check for voltage at the DLC, pin 16 is B+, 4 & 5 are still grounds, 6 & 14 are Interior CAN bus High and Low, the rest are labelled on the 1st OEM diagram, not sure how accurate the aftermarket one is I marked out in the 2nd diagram. But 6 and 14 should drop off to 0volts when it goes to sleep. I'm not sure how many CAN bus networks there are in total, since it looks like there are other diagnostic connectors possibly in the vehicle.

The label 6A on the amp clamp just means it will read up to 6amps on that setting, it's for accuracy if you need to measure higher amperage circuits.

These are the aftermarket power distribution diagrams, the OEMs are pretty difficult to read, but if something is missing let me know. The aftermarket info is not always full.
How many fuse panels are there in the vehicle?

These are the OEM diagrams for the interior fuse, and it looks like modules as well which offers coordinates to each component on the diagram. They look like they're designed more for engineers than technicians. But in case you need them, I will check the others as well. I think there's one more set with fuses.

These are the oem Front Back Up Fuse Box components, there are lists of connectors as well, they all look like this, you mentioned hearing a noise in the dash as well, check for a CD changer in the glovebox. Theres supposed to be one in there, maybe that has a CD stuck in it thats staying active. That happens pretty easily.

Try unplugging the cd changer, unless it's functioning okay. But it seems everyone I've ever had ended up with a cd stuck in it. But the Radio is on the CAN Bus and also has a right and left amplifier modules (so more computers to stay awake), an Amplifier module 2 Screen Antenna, an AM Trunk Lid Antenna amplifier, Emergency call system control unit, etc.
These are the OEM diagrams for the Radio, there are 7 modules that are linked to the Radio circuits.

What 2 fuses did you pull that made a difference so far?