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 body control module (BCM) internally grounds the door lock relay control circuit for the rear door lock relay when it receives a serial data message from the driver door module (DDM), passenger door module (PDM). When the BCM grounds the door lock relay control circuit the rear door lock relay is energized and locks the rear doors.