Circuit/System Description
WARNING: This page is about a different car, the 2010 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.
The window motors are supplied with B+ voltage which is used to power the motor and also used as a reference voltage within the logic circuit. When the driver or passenger window switch is pressed, a switch is closed causing the reference voltage in the appropriate circuit in the window motor to drop, the window motor responds by moving the window in the direction commanded.