So it's getting Cold...
Which is makes me bump the climate control system up on a laundry list of thing that need to be fixed on my 1983 300D. I pretty sure that the blower motor is the culprit, but I'm not 100% sure...
For a year or so prior my climate control worked (kind of) you could hit the defrost button and bamm hot Air, hit the open circle and it turned off. It started to go out and would only kick on every third time I hit the button and now it really will not come on at all. It makes a quick slight hissing noise as if it is trying to blow, right as I hit the button. Has anyone experienced this or can point me in the right direction? All advice is greatly appreciated
Stay warm-

You need to test the blower motor independently of the CCU.

Find the motor, disconnect the plug and apply 12V and ground to the plug. If the motor runs, the problem is likely the CCU. If the motor does not run, you found your problem.