One of my friends had a 120dB alarm speaker installed in the cabin of his car...hidden of course, so that when the car is attempting to get stolen the 120dB alarm is going off right in their ear. He had someone try to steal it and it went off and the guy bolted from the car and didn't harm anything.

My prior car had an alarm and I didn't know it so when I tried to jump my car it set off the alarm and I couldn't start my car for the life of me...it was almost impossible. I had to go under my steering wheel and rip out the brain of the system just to start it. That worked pretty good.

One downfall to a system installed wrong...my brother was travelling across country and his alarm system went off (had it wired to his starter) and killed his starter which in turned killed the car while driving. Just be VERY careful...and got a pro to do it...thats why they are trained