The best defense is a garage. If you leave a car on a city street for long, it will be broken into eventually. Most of the time it is a smash and grab as others post repliers had happen to them.

My car is not in a garage, but there is a light next to it and a camera watching it. If you are really worried about your car being stolen, get a GPS tracker like us private investigators use.

P.S. I am self upvoting only to get past the spammers.