Circuit/System Description
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The key cylinder switch contains a signal circuit and a low reference circuit. When the door key is turned to unlock, the switch contacts, which are normally open, close and the driver door module (DDM) detects a switched ground. The DDM sends a message via a serial data message to the passenger door module (PDM), and body control module (BCM) to lock or unlock their respective doors.