Door Ajar
WARNING: This page is about a different car, the 2017 Cadillac ATS. However, it is still accessible from the selected car via links, so may be relevant.
The body control module supplies a 12 V signal to the door ajar switch within the door latch assembly, when a door is open the door ajar switch closes pulling the 12 V signal low. When the body control module detects the drop in the 12 V signal circuit, it will then communicate with the instrument cluster via serial data message. The instrument cluster, upon receipt of this serial data message, will illuminate the appropriate door ajar indicator and also send a serial data message to the radio to activate the door ajar audible warning.