I have and 82 300SD. I have no blower motor. I get heat, but the blower never comes on. I replaced the control unit twice...each time it works for about 10 minutes then nothing. As soon as I switch it off or change to another button (from defrost) nothing works. I have also replaced the blower motor relay. I'm wondering if the motor is going bad (i.e. bad bushings) and is drawing too much power and somehow frying the control unit?

What else is possible? Is there another relay I don't know about? Regardless of the temp, the defrost should work right? Is there any way to test the relays?

No having a blower is getting real old.