California Vehicle Code _ 26708(a)(1) prohibits driving a motor vehicle
with any "object or material placed, displayed, installed, affixed,
or applied upon the windshield or side or rear windows." Therefore,
while enforcement may be lacking, it is nevertheless illegal to drive
your car with any object, including a radar detector, GPS unit, iPod or
sun shade, affixed to your window. Technically speaking, you can even
be cited for your windshield-mounted rear-view mirror!