GMC 2007 Sierra - Manual (Page 241)

Sensors
The solar sensor, located in the defrost grille, that
is in the middle of the instrument panel, monitors
the solar radiation. Do not cover the solar
sensor or the system will not work properly.
There is an interior temperature sensor located
next to the steering wheel that measures the
temperature of the air inside your vehicle.
There is also an exterior temperature sensor
located behind the front grille. This sensor reads
the outside air temperature and helps maintain the
temperature inside the vehicle. Any cover on
the front of the vehicle could cause a false reading
in the displayed temperature.
241