depending on whereyou live...I've heard lo-jack works great

http://www.lojack.com/

the best defense is a bright and fast blinking LED and The Club....if theives look and see that they have to bypass an alarm and hack off a club....thats enough time to slap a clip in the 45 and introduce yourself and your weapon to the thieve(s).....i also have a 120 lb rotty and she keeps people away when the car is at my house