I think you are saying that you have a 6 volt heater motor and a 12 volt system. If that device was a 3 terminal regulator and the ground came loose then it would certainly malfunction. I used a 1 ohm 50 watt resistor in series with my heater when I converted to 12 volts. Most of the truck part vendors sell them. They should be mounted on some sheet metal to dissipate heat.
