Blower motor issue

I would start with a charging system test. Depending on which HVAC system you have, there may be a computer involved, and they are very susceptible to low system voltage. If the alternator has a bad diode, it will only be able to develop exactly one third of its rated current. When the electrical system demands more than the alternator can develop, system voltage will drop. The head lamp system can draw well over fifteen amps. An electric fuel pump and the ignition system / injectors can draw another fifteen amps, and that can be more than an alternator with one defective diode can develop.

The clue here is the heater fan problem will not be affected by just the head lights. Anything that draws just as much current will also cause the same symptoms. Running the wipers and brake lights, for example, might do the same thing.

Your charging system can be tested at many popular auto parts stores, usually for free!

Let us know what you find out.

The Medic

Did they give you a printout? If they did, please post the results. I much prefer a set of numbers over "everything is okay". Some inexperienced mechanics see SOMETHING for output current and call that "okay". If what they saw was only one third of what should have been found, that is not okay.

Next, you need to specify if you have an automatic system or manually-operated controls for the functions. If you have to slide levers or have mechanical push buttons to change between defrost and heat, for example, you likely have a manual system with no computers. If you have tiny touch-buttons or knobs that turn to make the selections, there will be a computer in the system and those usually have a power module that runs the fan.

Did you check to see if the wipers or other circuits cause the same symptoms?

You can monitor the voltage going to the fan motor too, and watch what happens when you turn on the head lights. If the voltage stays constant but the fan stops running, it is a ground issue. The head lights are grounded under the hood and have nothing to do with the dash board, but there are interior lights that turn on with the tail lights, and some of those might share a ground with the heater fan or control module. Check the voltage on the fan motor's ground wire. If there is anything other than 0.0 volts, that circuit must be repaired.

When you had it tested the diodes were checked out too? Just wanna make sure a possibility was not skipped over, leaving you chasing your tail the rest of the time.

Got two more to check out

Yep, I know it's a lot of crap to look at, but maybe something will cash in.

This one is mine. Maybe it will be just that simple! Please see my answers in this link

https://www.2carpros.com/questions/1996-chevrolet-tahoe-wont-start-sounds-dead-battery-jumpbox-get-same-reults

The next two sorta go hand in hand.

This one is from the top of our page in "Repair Guides"

https://www.2carpros.com/articles/car-battery-dead-overnight

This is a way to see if there is a draw, I gave you the whole list, one may explain and show better than another.

https://www.youtube.com/results?search_query=parasitic+draw+test

Harbor Freight has a decent digital voltmeter for $5 (this is what I keep in both of my Jeeps and in the other family vehicles) Auto stores have 'em from about $15-$25.

Keep us posted

We can't have you all broke down!

The Medic

Can you get to the connection to the blower motor?

See if the plastic is melted and maybe the connectors are pushing/ moving back in the melted plastic, the connections are clean, the metal connectors are not burned, spread open not making good contact.

The Medic