Did you heard of the incident of stealing the sterio or a/c from a car? probably yes.
A car only has the glass window protection, which are good to save you from rain/wind/sun but not from human :-)
My idea is to device a metal/iron shielding in addition to glass window, when you are not in the car and your car is parked alone in the night time.
It is really going to save your expensive stereo and a/c from theft.