89 379 heater

Have you checked the fuse ?

 

Attach + and - wires to the fan. If it works then check the fuse (see above) and the original wires.

 

It doesn't have a rheostat. There are resistors next to the blower motor

Check the connection on the back of the control head. Those are prone to melting

Switch Harness, 5 Wire - Truck Air 11-3166, MEI 1055H

 

I forgot to mentioned the switch. On my former 94 379, I had to place the switch because it wouldn’t work on high. When I removed the old switch, it seems it has gotten hot, had to snip the end of the wires and put new connecting ends. Have you replace the switch yet ?

 

One of the reasons switch’s and resistors give problems is because the 379 is notorious for getting moisture in the fuse box causing resistance and it likes to melt the connections at the switch and resistor
Especially if the windshield has ever leak which a fair amount of them do