Bumping this thread as I'm dealing with a similar issue in my '85 300D. I have played with the HVAC buttons on cold car, warm car, off car, running car..

I removed the blower motor this afternoon and looked around it. I'm not sure how to disassemble to the point that the brushes are accessible.

Anyway, I put the motor in my bench vice and hooked it up directly to 12V source. It not only spun, but spun with some gusto! The motor is definitely not the problem.

I also tried measuring power at the plug for the motor (connects to the motor under the dash). FWIW, I'm an electrical idiot, but this seemed to make some sense. I stuck my multi-meter leads into the female ports of the plug and tried various defrost/fan speed settings with the key in the 'on' (not running) position. For all of these tests, I showed 0.00 on the meter. Not sure if I did this correctly or not. I did try swapping the leads from side to side to make sure they would have been in the correct position.

I have tried jiggling the key in the ignition with the key in 'on' (not running) position, and cycled through the HVAC setting. No dice.

Fuses are good.

When I press one of the HVAC buttons on the CCU, I hear a click somewhere in the engine bay (near the firewall? Hard to tell). So, to my small mind, that tells me there's some signal going from CCU and getting lost somewhere in the middle? Dunno, could be wrong.

I see that testing the resistor on the PS fender under the hood is an option. Same for the sensor (or switch?) on the thermostat. I'm not sure how to test these. I pulled the plug from the resistor and stuck the positive end in each pin-hole, grounded the negative, and turned the defrost 'on'. I got 0.0 for each pinhole. Again, no clue if this is an adequate measure of testing the resistor.

I removed and disassembled the CCU today. I did not notice any solder points that looked corroded, cracked, burned, broken, etc. Truthfully, I was surprised to see how nice it actually was inside. I have emailed JamesDean to ask about testing/repair services (he's about to get my cruise amp and actuator as well).

I do not have an Aux pump. I have deleted the monovalve system completely and that went with it. Worth noting, the HVAC has worked beautifully with the monovalve delete for over a year.

Any specific, detailed, advice is highly appreciated. I can do a ton of my own work, but with electrical specifically, just pretend I'm an idiot.

Thanks.