I'm still trying to diagnose my heater. 1980 F-150, blower motor only runs on the high speed setting. I looked at the wiring diagram in my manual, and I see that the wire from the fan speed switch goes through a resistor mounted in the blower housing, then on to the motor. the resistor is what makes the motor run at different speeds. Does anybody know how to test this resistor ? I've got a meter, but I'm not sure how to run the test, if it can be tested. Do I use the Ohms (continuity) test? I'll check with the ford dealer today. The resistor looks like a separate part, but it might be built into the motor itself.