I'm not too sure about the 78 but on my 85 there is a pressure switch and a temp. switch on the drier.

Crossing the leads on the temp. switch should cause the aux. fan to run with the key on. But, like I said. That is on the 85.

I'm sure someone on here has a wiring diagram for your car. That system is pretty easy to troubleshoot.

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AJ

1985 300D (SOLD)