GM Service Manual Online
For 1990-2009 cars only

With the Ignition Switch in the RUN position, voltage is supplied by the HTR-A/C fuse #12, through CKT 141 (BRN) to the Rear Window Defogger Switch at the HVAC Control Module. When the Rear Window Defogger Switch is pressed, the contacts close, which provides voltage to the Rear Defogger Grid through CKT 293 (PPL) and the LED Indicator in the switch, which is provided ground through CKT 150 (BLK) to ground G200. The Rear Window Defogger Grids will heat to remove fog from the surface of the window. Ground for the defogger grids is provided through CKT 150 (BLK) to ground G400. (If equipped with Cargo Doors the doors must be closed in order to provide contact for the power and ground operation).

The Solid State Time Delay Relay, which is supplied voltage from the RR-DEFOG fuse located in the Underhood-Relay Center passing through CKT 1440 (ORN), will automatically turn-off the defogger grids and the LED after approximately 10 minutes. Depressing the push-button again, will re-initiate the timed operation. The defogger grids are also automatically turned off if the Rear Defogger Switch is pressed again or the Ignition Switch is turned to the OFF position.