Do you have a main electric fan in the place of the engine fan? Or, are you talking about the ACC fan (In front of the condensor)?

The deforster takes a boat load of power, as does any resistance heat device. If you a multimeter or voltometer, check the battery voltage:

Engine off
Engine at idle, no loads
Engine at 2000rpm, no loads

12 with engine of is normal.
With the engine on, it should be between 13-14.5v