Since I park at a high school and would have an absolute episode if someone were even leaning up against my car with jeans on, I checked out the possibility a while back. I contacted "Fritzauf" and he has a Compustar system. (http://www.compustar.com/)

I also know that Viper makes a good system. I think about $450 is what a top of the line Viper alarm would cost.

[MEDIA]http://www.youtube.com/watch?v=2OQk3XjLPvU&feature=related[/MEDIA]

Now, I what I would be interested in was a proximity sensor that was wired into the factory alarm. I wouldn't think that this would be too difficult to do. That's what I would be sold on. Good luck. Maybe you should try to contact "Fritzauf."