When the fan is working, it works fine on all speeds, but sometimes it won't work at all. If I've left the switch on, but the fan is not working, it might come on when I go over a rough patch in the road. Should I be looking at the motor, switch, or resistor - or some combination thereof?

How do I make the diagnosis?

 

Pull the resistor, look for broken coils. Replace resistor if broken coils are present. Test fan.