GMC 2010 Yukon/Yukon XL Owner's Manual page 247

The rear window defogger uses a warming grid to
remove fog from the rear window.
rear window defogger on or off. It automatically
turns off several minutes after it has been activated.
The defogger can also be turned off by turning off the
engine. Do not drive the vehicle until all the windows
are clear.
For vehicles with heated outside
rearview mirrors, the mirrors heat to help clear fog
or frost from the surface of the mirror when the rear
window defog button is pressed. See Outside Power
Mirrors on page 3 55.
Press to turn the
The solar sensor, located in the defrost grille in the
middle of the instrument panel, monitors the solar heat.
Do not cover the solar sensor or the system will not
work properly.
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