A rhiostat is a variable resistor - wire wound with a wiper that rotates to pick off a voltage - similar to the control to dim the dashboard lights - infinitely variable within its range. Your car heater fan may have a rhiostat but heaters usually have fixed positions like Off-1-2-3-4. No particular need for an expensive rhiostat. If your heater does have a rhiostat it must be connected to a rotating control knob.
Peccavi, October 2008
