GMC 2012 Sierra Denali Owner's Manual page 248

8-6 Climate Controls
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.
Heated Mirror: 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 Power Mirrors on
page 2 13.
Sensors
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.
The interior temperature sensor,
located in the headliner, measures
the temperature of the air inside the
vehicle.