Car heater circuit layout

The heater fan is simply an electric motor mounted inside the heater unit. Current is fed to the motor via a switch on the dash.

If your fan has several speeds, they are usually controlled by a large resistor block mounted piggy-back on the fan motor. For slow speeds the dash switch directs the current to the motor via the resistor block. For top speed the switch bypasses the resistor and applies full current to the motor.