I'm unsure about the legality of shooting someone who's breaking into your car. Sure, they're comitting a crime... but I think you're legally required to try and verbally "dissuade" them first. If you just start shooting, things get messy. The guy can just claim he thought it was his car out there in the dark (yeah right) and was wondering why his key didn't fit, the next thing he knew, he had a bullet in his leg... you know?
Which is why I've always wished they had a car security system where either the shifter or the steering wheel became electrically charged... something really space-age, like you put your thumb-print on a little scanner, and if it doesn't match, BAM, 100K volts to the shifter and steering wheel. That should be enough juice to make its way through leather gloves... yow!
Hmmmmmmmmm.... if it doesn't exist, it should...