You will find that the car alarm will go off too often to be of any use like this. You'll need to add some circuitry to discriminate between a neighbor closing a car door, cat jumping on your car, Harley motorcylcle passing by. Otherwise, the alarm will be just as annoying as the stereo. The alarm would need to be considerably louder than the stereo, otherwise it would be drowned out. People who listen to "music" (RAP os short viresion of CRAP) that loud, usually aren't to concerned about quality or enjoyment. Almost all of those ******** driving those bass-shaker-mobiles I've seen at stoplights, are wearing earplugs.
Seriously, if you are in the United States, call the police. They will tell you what hours they can get away with disturbing your peace. Get some of the other neighbors involved. One neighbor calling in a complaint may not carry enough weight, but two or three, on the some night will end it quickly. If there are drugs involved, it won't take long for them to move on.