a rheostat is a variable resistance, works the same on 6 or 12V, and your tractor may have had one used to adjust the voltage to something [coil, points?], when it fries, the circuit is open so whatever doesn't get any power - if you just put a jumper across that rheostat you'll prolly be able to locate what it feeds from the smoke
and the heater controls aren't really rheostats, they're variable dropping resistors, with taps, usually 2 or 3 position items, a rheostat is "infinitely" variable from none to maximum, like the volume control on yer radio
the heater blower is about the highest stock amp draw in these old trucks, and those heater "rheostats" wouldn't cut the voltage enuff to keep a 6V motor from over-revving .... note how much better/faster a 6V starter works on 12V - for the seconds that it's needed ... as Spencer said, the heater fan is needed for extended periods
Bill
