A number of car audio systems that I have seen have removable face plates. Thief looks in, sees that the faceplate has been removed and will then realize the unit is worthless to resell and will move on.
I had two faceplate-less car stereos stolen. Turns out that they know that most people leave the faceplate in the glovebox. Some other crims are too stupid to know that you can't just walk into a stereo shop and buy a replacement faceplate. (I was in a stereo shop for two hours and no fewer than three idiots came in with a bare stereo looking to buy a faceplate.)