GMC 2007 Sierra 1500 Pickup Owner's Manual page 234

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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 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.
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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 the vehicle
has them.
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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.
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(Mirror): If your vehicle does not have a rear
defogger and has 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.