Blower Motor Speed
WARNING: This page is about a different car, the 2013 Ford Fiesta. However, it is still accessible from the selected car via links, so may be relevant.
The HVAC module blower motor speed selector varies the blower motor ground circuit resistance by adding or bypassing series resistance in the blower motor resistor. Increased resistance lowers the blower motor speed, and lowered resistance increases blower motor speed. When the blower motor is in the HI (4) position, the blower motor resistor is bypassed.
The blower motor resistor is located on the heater core and evaporator core housing below the glove compartment.