First of all hiding everything is your best bet(Fold-away screen, consealing door, tint, stealth trunk mounting). And consider the effect of having car alarm decals discretly on two or so windows and a blinking red LED.
Sometimes criminals do not want to put for the effort when the next car is without an alarm (after market anyway). Third, if they make the effort to break the window, sound an ear piercing piezo siren. Hidden deep within the dash, the alarm is triggered no one wants to enter the car with 110db 3khz siren blaring. Infact, my install will feature such a siren inside and out powered off of the second battery as well as a strobe to illuminate the night. As far as retaliation goes, no way to win that one. And with all systems designed by hoomans, there is a chance of back fire.
Cats also don't like loud noises and blinking lights..just like the neighbors. The shock idea is cool, but not safe nor practical at this point.
If I had my way, next thief gets to try his luck with my power antenna/tesla coil.
Also some radio active goo to making tracking down get-aways easy with my giger counter.
