GMC 2006 Sierra - Manual (Page 202)

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(Defrost): This mode directs most of the air to the
windshield and the side window outlets, with only a little
air directed to the floor outlets. The air conditioning
compressor will run automatically to dehumidify the air to
prevent window fogging. The recirculation mode cannot
be selected while in defrost mode.
Do not drive the vehicle until all the windows are clear.
Rear Window Defogger
If your vehicle has this feature, the lines you see on the
rear window warm the glass. The rear window defogger
uses a warming grid to remove fog from the rear window.
If your vehicle has a sliding rear window, you will not
have a rear window defogger; however, your vehicle will
still have a mirror button to activate the heated outside
rearview mirrors, if equipped.
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(Rear): Press this button to turn the rear window
defogger on or off. An indicator light in the button will
come on to let you know that the rear window defogger
is activated. This button will also activate the heated
mirrors, if equipped.
The rear window defogger will automatically turn off
several minutes after the button is pressed. Pressing the
button while the defogger is running can also turn off the
defogger.
W
(Mirror): If your vehicle does not have a rear
defogger and is equipped with heated mirrors, your
vehicle will have this button to activate the heated
mirrors.
Notice: Do not use anything sharp on the inside of
the rear window. If you do, you could cut or damage
the warming grid, and the repairs would not be
covered by your warranty. Do not attach a temporary
vehicle license, tape, a decal or anything similar to
the defogger grid.
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