HVAC Control Module
WARNING: This page is about a different car, the 2002 Buick Regal and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
Rear Defog Output: The scan tool displays On/Off. When the rear window defogger switch is depressed, the HVAC control module energizes the Rear Window Defogger relay by grounding the control circuit of the relay, which the scan tool displays as On.
Rear Defrost Button: The scan tool displays On/Off. When the rear window defogger switch is depressed, the scan tool will momentarily display On.