6v to 12v step up converter

I Feel your pain. I have a 53Plymouth 6v positive ground vehicle as well and I went through this trying to get a 12 volt aftermarket radio to work. Had to go to an anodized aluminum bodied casing for the inverter I got from JC Whitney. I would think that any inverter you use for something that draws as many amps as an electric heater will surely kill your six volt battery. I would retrofit a different core and replace the motor (if bad) with another six volt motor. You can isolate the mount of the motor to reverse the polarity if need be.