GMC 2002 Envoy - Manual (Page 183)

3-9
Rear Window Defogger
The lines you see on the rear window warm the glass.
Press the button to start warming your window. A light
will glow in the button while the defogger is working.
If your vehicle is equipped with heated mirrors, this
button will activate them.
After 10 minutes it will turn off by itself, or press the
button during the heating cycle to turn it off. If you need
additional warming time, push the button again.
Do not attach a temporary vehicle license, tape or decals
across the defogger grid on the rear window.
NOTICE:
Do not try to clear frost or other material from
the inside of the rear window with a razor blade
or anything else that is sharp. This may damage
the defogger grid. The repairs wouldn’t be
covered by your warranty.