Circuit/System Description
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The power door lock switch signal circuits are referenced from battery voltage by the body control module (BCM). When a switch is pressed to the lock or unlock position the appropriate signal circuit is grounded. When a signal circuit is grounded the reference voltage goes low indicating to the BCM that the corresponding switch is active.