Does anyone know what the little black antenna on the driver's side roof side is for? Radio? Onstar? Pigeon proofing?

 

The part that sticks up is the cellular antenna, and the base of it has a GPS antenna in it, as well as an XM antenna(If equipped)

 

Great, so that means that the old school mast up front coming out of the right side cowl is the radio? I was hoping that I can use that small antenna for the radio instead. I also have the cell antenna on the left rear glass. What is that for?