GMC 1994 Yukon Owner's Manual page 141

Air Vents
If your vehicle does not have air conditioning, there are air vents below the
instrument panel.
Use the handles below
your steering wheel,
with the vent symbols
on them, to open and
close the vents.
If your vehicle has air
conditioning, you will
find air vents in the
center and on the sides
of your instrument
panel.
You can move the vents grilles from side to side or up and down 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.
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