My mother recently purchased an 08 HHR LS, it did not come with the same HU that my grandmother's 08 HHR came with. My grandmother got the most expensive package that came with the HU with xm controls/3 month free xm. It does look like my mother has 2 antennas, possibly one for XM?

What I am looking for is the easiest way to install xm into her car. Is the antenna on the back of the car in fact wired to the front of the car for xm? Any help beyond this would be great, thanks.

There is a little black bubble on the rear roof of the car. If it is equipt with XM, thisbubble will contain a GPS antenna for OnStar, XM ANtenna, and the little mast sticking out is the cellular antenna. If it is not equipt with xm, it will look exactly the same, but will not have the XM antenna built in