Final stage resistor (FSR) or final stage unit (FSU). It regulates the fan speed coming from the blower motor. If your blower motor seems to have a mind of its own the blower resistor is the first part to check or replace. A failed FSU will cause the fan to not work at all, only work on one speed, or alternate the fan speed on its own. It may even cause battery drain as the blower runs even with the car turned off.
