Also, I just looked in the manual, it's on page 51.
Interior and exterior mirrors, automatic
dimming feature*
The automatic dimming feature of the
interior
and
exterior mirrors* is controlled by two photo
cells in the interior rearview mirror. One photo
cell is in the mirror frame, see arrow; the other is
on the back of the mirror.
In order to ensure that the system functions
correctly, keep the photo cells clean, do not
cover the area between the interior rearview
mirror and windshield, and do not affix adhesive
labels or stickers of any kind to the windshield
directly in front of the mirror.