The control unit is the part where you move the switches.

 

On the models with ATC, the blower resistor is actually a little control module so it can produce any blower speed it wants instead of the 4 speeds on a manual system. When the blower motor draws too many amps from the bushings getting wet it causes the module to fail in a way that causes the fan to run at high speed any time the key is turned on. The control head can be turned on or off and the blower will continue to run on high speed. The resistor module is around $100 so if I were you, I would consider replacing the blower motor also since the chances are very good that it is the cause of the failure in the first place.

 

that is the PERFECT answer. too bad the posts didn't start with this!. (gave the scenario to why it fails, how it fails, where it's located, how much)