If you use a component that is made to reduce 12V to 6V and it will handle your amp draw and it smokes a simple speed switch, you are missing something, it defies logic or have a bad motor or switch or wiring. You can go off into electronics land if you want to but this is pretty simple. Did you measure to see you were getting 6V and any amp draw changes at speed levels in your "experimental" efforts? The tech tip section here says use a voltage reducer. This is common practice and the ones linked are designed for automotive use and exactly what you are doing. They will reduce the voltage from 12 to 6, two in parallel will handle up to 8A.(assume your mention of 5.75A is max draw). So that set up will work if other components are working properly. The switches will be hot at mid range, Very hot. Normal. (Not smoking hot.) The resistor is dissipating the energy that would go to motor because it is causing motor to operate slower. If it smokes, again, bad switch, motor, wiring or all three. Are you using the word smoke as just an expression? Do you have the heater blower motor/switch fused? If fused, it will blow before smoke. Fuses are for protecting switches and fan motors, etc. Make sure fuse is not too big. I see you took out motor, did you check fins, rotation, clogging, rubbing, wiring, worn brushes, dry on oil?????? I see you got it running backwards, then "fixed it", I see it's a GMC heater someone put in. Is this a Chevy or GMC "first series"? Methinks you better redo this heater motor and bench test before installing.
