The best protection against people trying to steal your car audio equipment is to NOT LET PEOPLE KNOW YOU HAVE IT
Its always the guys that "pimp" their stereo and do NOT turn it down when coming into their neighborhood/school/work that usually find their stuff missing. I may talk about my audio mods on this board, but to the people around my area, my car is as quiet as a mouse. Its sad that I can't listen to my **** whenever I want because I have to protect it, but needless to say, we've invested thousands of dollars into the system, and I still have it all...