GMC Sierra Owners Manual
Rear Window Defogger
For vehicles with a rear window defogger, a warming grid is used to remove fog from the rear window.
(Rear): Press to turn the rear window defogger on or off. An indicator light on the button comes on to show that the rear window defogger is on.
The rear window defogger only works when the ignition is in ON/ RUN. The rear window defogger stays on for approximately 10 minutes after the button is pressed, unless the ignition is turned to ACC/ACCESSORY or LOCK/OFF. The defogger can also be turned off by turning off the engine.
Notice: Do not use anything sharp on the inside of the rear window. If you do, you could cut or damage the warming grid, and the repairs would not be covered by the vehicle warranty. Do not attach a temporary vehicle license, tape, a decal, or anything similar to the defogger grid.