I have a 78 450sl. Air conditioning, blower motor has worked flawlessly until a couple of days ago. Air conditioner stopped cooling, now when I flip the air conditioning compressor switch off, the blower motor stops, then in a few seconds will start blowing again. I can flip the compressor on, still will not cool and there is no affect on the blower, flip it off, and again the blower will stop, and again come back on after a few seconds.

 

You might need to replace the blower motor resistor.