This is a 2006 custom luxury. I haven't really noticed the box on other models on the road. Trunk<br ><br >

 

It's an antenna for XM satellite radio. the '04 - '06 LS models were satellite radio "ready." Meaning, you had to add the satellite radio tuner accessory package (with antenna) to have access to satellite radio features.

 

And since sat radio sucks in both value and audio quality, you need not concern yourself with it.

It's actually a supplemental antenna. Most users find the shark fin (or wherever the stock antenna is) is enough, but if you begin to get excessive cut-outs from bad reception, they install this one which is supposed to be better at bringing in the ****ty programming and compressed to hell music.