1999 GMC Sierra 1500 Pickup Owner's Manual - Page 168

Comfort Controls & Audio Systems Rear Window Defogger You cantell if yourvehiclehasthisoptionbylookingattherearwindow. youseelinesrunningacrosstheglass,youhaveit.Thes'earethewires whichheatyourwindow. If For best results, clear the window ofasmuchsnow or ice as possible first. - To turn on the rearwindowdefogger, find theswitchmarked REAR DEFOG onyourinstrumentpanel,justbelowtheheatersystem.Presstherightside it, untilthelight in theswitchcomes of the switch,withthedefogsymbolon if the ignition on, then release it. Therearwindowdefoggerwillonlywork switch is turned to RUN. You can turn thedefogger off atanytimebypressingtheleft,lightedside of off after several minutes, so that the the switch. The defogger will shut itself glass does not get too hot. If thedefoggershuts off, andthewindowstill isn'tclear,turnthedefoggeronagain. 3-0