To add GM XM to an HHR you need to check your RPO codes in the glove box. The criteria is:

XM Radio Upgrade Kit (w/ RPO Code UE1, w/o RPO Code U1C)
Part Number 17800594

 

XM is nice, but I suggest you keep a close eye on your billing. I have been trying to get my bill straightened out for a while now. The put a monthly charge on my credit card for a radio that was prepaid for 3 years, when I call about that radio they increased the monthly fee for one of the others and the list goes on. I finally started cancelling subscriptions and when the main office called and wanted to know why I was dropping their service I think me finally got things back right. However, I will not turn the subscription back on that I cancelled.