Body Control Module (BCM)
WARNING: This page is about a different car, the 2002 Cadillac Eldorado. However, it is still accessible from the selected car via links, so may be relevant.
Rear Defog Module Output: The scan tool displays ON/Off. When the rear window defogger switch is depressed, the HVAC control module sends a class 2 message to the BCM to enable the rear defogger relay. The BCM energizes the rear defogger relay by grounding the control circuit of the rear defogger relay, which the scan tool displays as On.