Key Cylinder Lock
WARNING: This page is about a different car, the 2004 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
The BCM will lock all of the doors immediately upon detecting a door lock cylinder switch in the Lock position. The BCM supplies battery positive voltage, through the door lock key switch lock signal circuit, to the door lock cylinder switch. The door lock cylinder switch is grounded, through the ground circuit, to a body ground stake. When the door lock cylinder switch is activated to the Lock function, the normally open contacts of the two circuits are closed, and the BCM interprets the flow of voltage as a lock doors input.