1998 GMC Suburban Owner's Manual - Page 171

Rear Window Defogger (If Equipped) If you see lines running across the rear window, you have a rear window defogger. The lines warm the glass. To turn on the rear window defogger, press this button next to the fan control knob. The rear window defogger will only work if the igllition switch is turned to RUN. For best results, first clear the window of as much snow or ice a s possible. If your vehicle is equipped with the optional heated outside rearview mirrors, the rear window defogger button will activate the. rear window defogger and the heated outside rearview mirrors. Do not attach a temporary vehicle license, tape or decals across the defogger grid on the rear window. L -~ NOTICE: Don't use a razor blade or anything else sharp on the inside of the rear window.If you do, you could cut or damage the warming grid, and the repairs wouldn't be covered by your warranty. The defogger will shut itself off after several minutes. If you need additional warming time, press the button again. You can turn the defogger off at any time by pressing the button. 3-9