oldblu,
I am not real close to being an electrician either, but I have been called a know-it-all, so here is my unprofessional, uncalled for opinion. I don't think a automotive device like a motor would be built to ground through the case.
I think one wire is ground and the other is power. I think the speed or speeds of the motor are determined by the voltage applied to it. If it is a two speed motor, the switch will have the appropriate resistor built in to reduce the voltage for the slower speed. That is why you see the large resistors in the factory switch in the link I supplied.
You are sort of dealing with an unknown...a $5 dollar motor you picked up at the salvage yard that looks the same as the one you took out of your truck!
I would think it much easier, from a manufacturing standpoint, to put the speed regulation in the switch rather than the motor. It wouldn't cost much to try wrenchbenders switch suggestion, and that might be the answer.
Stuart
