GMC 2000 Sierra - Manual (Page 175)

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If your vehicle is equipped with heated outside mirrors,
the rear window defogger button will also activate the
heated outside mirrors.
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 VENT to direct outside air
through your vehicle. Air will flow through the
instrument panel outlets.
Your vehicle’s ventilation system supplies outside air
to the inside of your vehicle when it is moving. With
the side windows closed, air will flow into the front air
inlet grilles, through the vehicle, and out the air
exhaust valves.
Outside air will also enter the vehicle when the heater or
the air conditioning fan is running, unless you have the
RECIRCULATION button pushed in. For more
information on the RECIRCULATION button, see
“Air Conditioning” earlier in this section.
Your vehicle has air
outlets in the center and
on the sides of your
instrument panel.
You can turn the outlets from side - to - side or up and
down to direct the flow of air. The left driver and
passenger outlets on vehicles equipped with air
conditioning have shut off knobs. Turn the knob to the
left to shut off air flow, to the right to allow air flow.
When you close an outlet, it will increase the flow of air
coming out of any outlets that are open. The center
outlet does not completely shut off airflow.