Terry may have the issue nailed. My 72 had all the classic symptoms discussed---low, med--no high. Changed switch, relay, resistor, redid relay ground etc. Then noticed this big orange wire that was supposed to be hooked to the rear of the cig lighter. Once hooked up with a new fuse---high speed fan works as designed.
Appears them little electrons go round and round in them wires till they hit something they like to work. That's probably why you can add power by straightening out wires so them electrons don't get dizzy goin in circles etc.
Seriously, check that cig lighter fuse, grounds etc. Good luck-Huck
