GMC 1999 Sierra 1500 Pickup Owner's Manual page 163

Air Vents
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You will find air vents in the center and
on the sides of your instrument
panel. You can move the vents to direct the flow of air, or close the vents
altogether. When you close a vent,
it will increase the flow of air coming out
of any vents that are open.
If your vehicle does not have air conditioning, there are air vents below the
instrument panel. Each vent has a handle with a vent symbol on it. Use the
handle to open and close the vent.
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