Security lights come on by themselves

The wiring to the horm was probably cut but the alarm system is probably still intact. Sometimes you can find the brain to the security system behind the rear defrost button. If the button acts somewhat odd, not quite sititng on the right command but somehow working anyway suspect the car alarm control panel has been hidden there. Unplug the brain and plug the wire to the rear defrost back in so the rear defrost will continue to work. That should do much to dismantle your alarm. This may not work for all vehicles but for some. Try it if you like.