I know this thread is an old one but thought I would post an update in case anyone else searches for answers to this question. I have a recirculating dealer installed Harrison heater in my 54 3100. I replaced the 6v motor with a 12v motor. I took the truck on a 1 1/2 hour ride with the 4 position (OFF-LOW-MED-HIGH) original rheostat switch set to LOW. It worked perfectly. I did not increase the size of the fuse. I stayed with the original 20 AMP size. To be clear, I did then try the other speeds and the switch performed as it should.
