GMC 2000 Envoy Owner's Manual page 143

Rear Window Defogger
For best results, clear the window of as much snow or
ice as possible before using the rear window defogger.
To turn on the rear window defogger, find the button
marked REAR with the defog symbol on the lower right
corner of your climate control system. Press the button
until the light in the switch comes on, then release it.
It will only work if the ignition switch is turned to RUN.
Pressing the REAR button on your climate control
system will also activate your heated outside mirrors.
This will help to keep ice, snow and condensation
from collecting on your mirror surfaces.
3-6
The lines you see on
the rear window warm
the glass.
The rear window defogger will stay on for five to seven
minutes, then turn off. If you need additional warming
time, turn it on again. You can turn the defogger off at
any time by pressing the button.
Do not attach a temporary vehicle license, tape or decals
across the defogger grid on the rear window.
NOTICE:
Don't use a razor blade or something 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.
Ventilation System
For mild outside temperatures when little heating or
cooling is needed, use PANEL with the OUTSIDE AIR
button enabled to direct outside air through your vehicle.
Airflow is through the instrument panel outlets.
Your vehicle's ventilation system supplies outside air to
the inside of your vehicle when it is moving. When the
vehicle is not moving, you can get outside air to flow
through by selecting any mode and any fan speed.