You need an aftermarket Two Way Alarm System with a microwave perimeter sensor. I've got one and it works great. I installed it as a stand alone system in addition to the factory alarm system (the factory alarm is not bypassed).
If someone get close to the car it gives three warning chirps and sends a chirp and message to my remote to let me know what's going on.
O.K. you have to set both alarms (if you desire) and you may get some extra exercise running out to your car all the time, but you will have peace of mind knowing what's going on when you're away from your baby.
And just think of the fun you'll have if you actually catch someone in the act!!!

PEACE

Or you can get one of those Trunk Monkeys.