I have one of the 6 V repro heater switches on my truck, which is still 6 V. The first new one I had melted when I tried to use it on anything but full power. I only use it on full power which means the resistor isn't used. I get the impression that the new switch resistors can't handle the current from the older 6V motors.
Have you replaced the heater motor with a 12 V? If so it might not be as bad. I must say that the amount of heat given off by my heater is quite minimal. The core is new and I use a 160 thermostat.
By the way, in case you haven't noticed, this is posted in the Radio Forum. I can move it if you like.
