Thrift-Air Heater issues... '64 C10

[Streetglide2015]

Could this be a resistor problem? Need your expert advice...
I keep replacing fuses in the block, for the heater. The switch seems to work fine (all three fan speeds) each time I replace a fuse. HOWEVER, the fan seems to blow fuses when I rev the engine a bit, and the speed of the fan increases with acceleration.
Based on some of the things I have read about the Thrift-Air system, it sounds like a bad resistor (?) There seems to be no other electrical issues with this '64 C10, but could it also be a voltage regulator causing this?
I know enough to be dangerous, about electrical issues... Any advice or opinions are welcome! Thanks!