Usually the fan is controlled by some form of temperature sensor within the unit so that control of it cannot be bypassed. It basically turns on whenever heat is sensed and doesn't turn off until the unit drops below the sensor's preset cutout temp.
This setup would require that the unit has power at all times and that the thermostat only controls the heating element. If it is tripping like you say (assuming you mean tripping the breaker), then there is more of a problem here than just thermostat wiring.
