I drive a 1990 740 wagon and since the summer has come, I've realized the AC doesn't work which reminded me of the fact my heated seat doesn't work. Power gets to the switch as the light turns on in the switch. How can I tell what parts actually work? thanks!

 

I generally debug electrical things with a volt meter... follow the circuit and check for expected voltages at each stage.

as far as your A/C, that could be almost anything, you haven't given us much to go on.

 

ive already diagnosed my ac, it needs some more refrigerant, it blows cold just not very well. I'll check for voltage at the connector on the seat