Chevrolet 2007 TrailBlazer Owner's Manua page 194

Defogging and Defrosting
Fog on the inside of the windows is a result of
moisture condensing on the cool window glass.
This can be reduced if the climate control system
is used properly. You can use either defog or
front defrost to clear fog or frost from your
windshield.
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(Defog): Use this setting to clear the windows
of fog or moisture. This setting will deliver air
to the floor and windshield outlets.
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(Front Defrost): Press the front defrost button
to clear the windshield and side windows of
frost or fog quickly. The system will automatically
control the fan speed if you select defrost
from AUTO mode. If the outside temperature is
40°F (4°C) or warmer, your air conditioning
compressor will automatically run to help
dehumidify the air and dry the windshield.
Do not drive the vehicle until all the windows
are clear.
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Rear Window Defogger
If your vehicle has the rear window defogger a
warming grid is used to remove fog from the
rear window.
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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.
The rear window defogger will turn off
approximately 10 minutes after the button is
pressed. If you need additional warming time,
press the button again.
If your vehicle has heated mirrors, they will come
on when this button is pressed.
Notice: Do not use a razor blade or sharp
object to clear the inside rear window. Do not
adhere anything to the defogger grid lines
in the rear glass. These actions may damage
the rear defogger. Repairs would not be
covered by your warranty.