oldbluetruck,
I would make sure the new rheostat and fan motor were rated for the same voltage as your truck (your battery). The rheostat would go in series on the + or supply side of the motor and the other motor lead would be grounded.
The switch should have an off position, as well as being variable. It will get warm, but I don't think it is suppose to glow like a light. Don't forget to put a fuse in this circuit...didn't some of those switches have one hooked on the back?
Stuart
