1994 GMC Suburban Owner's Manual - Page 149

Rear Window Defogger r o u can tell if your vehicle has this option by looking at the rear window. If you see lines running across the glass, you have it. These are the wire-like elements which heat your window. NOTICE: Scraping the inside your rear window could cut and damage of the defogger.Your warranty would not cover this damage. And, don't put decals there, you might have to scrape themoff. For best results, clear the window of as much snow or ice as possible first. To turn on the rear window defogger, find the switch marked REAR DEFOG on your instrument panel, just below the heater system. Press the right side of the switch, with the defog symbol on it, until the light in the switch comes on. then release it. The rear window defogger will only work if the ignition switch is turned to RUN. You can turn the defogger off at any time by pressing the left, lighted side of the switch. Thedefogger will shut itself off after several minutes, so that the glass does not get too hot. If the defogger shuts off, and the window still isn't clear, turn the defogger on again. 3-11