If you live in the area where you need to worry about things like this I would consider moving... I did.
The best deterrent is to have nothing of value in the car (including stereo) and have no "covers" so that the entire inside of vehicle is visible from the putside.
This I heard from the source - a man who served time for breaking into cars.
Most sought after items (by thieves): high end stereos and speakers, purses, briefcases and laptop computers. Oh and cellphones.
All it takes is a half inch ball bearing ball and you can get inside the car with virtually no noise at all.
Life is what happens to you while you're busy making other plans. - John Lennon