I find it humorous how many people go out and buy expensive car alarms and they don't help at all...I've lived in both a big city and small town and when people hear a car alarm going off, they don't do anything!!!! The best thing you can do is DON'T cover your car with car audio stickers! And don't go around bragging to everyone you pass in the street about how great your car stereo is...this is the biggest problem for a lot of high schoolers (at least when I was there). You should buy your stereo for you, not to impress other people, and if you decide to get a nice stereo for other people, thats probably who will end up having it! You could always go out and get the ugliest, old ladyish car you can find to put your stereo in, make sure you leave the luggage rack on and keep the ahfgan covering the back seat...not many stereo thieves look at those cars looking for high end audio. But most of us don't want to go as far as taking THOSE measures!!!
Keep it loud!
JELLYDIME