A lot of HHR owners have the correct head unit radio (has the "Cat" button) but did not get the XM option.
Now you can add XM to that radio, have the head unit display the song/artist/channel information, and use the head unit to control the XM tuner...ALL just like the factory installation.

Here are the folks selling the system:

http://stores.ebay.com/Integration-Solutions

No, I have no interest in the company, but last year a lot of members were looking for a way to add XM or Sirius using their head unit for control.

Have fun!