I had a friend back in high school that used to go "car shopping", he and his buddy never messed with cars that appeared to have quality factory/aftermarket alarms installed.
The last thing theieves, any theives, want is to be detected, or have something alert people to there prescence.
Funny story years ago my roommate kept getting his paper stollen, so I set up a camera on the parking lot with the live feed to my desktop. There i am chilling late one night, (blinds closed, lights off,) surfing the web when i notice movment in the top corner of the monitor. Popped on the live feed tab and sure enough theres some guy with a crowbar trying to jimmy my door open. Within 3 seconds i threw the window open and shouted "ARE YOU FUCKING SERIOUS?" - Scared the living shit out of him, never seen anyone run so fast in my life.:lolguy: