It is the transmitter/pager for my car alarm. There is another one above that one, but you don't see it in the picture. And that one is the shock sensor for the windshield. My OnStar antenna is the shark fin in front of my sunroof.
Injun #4 that is a good idea to scrape off the "AVS" logo on the tailcovers. I might have to do that one of these days real soon.