6 DISC STEREO mcss XM
I was the first to go aftermarket, b/c the dealer promised to add factory XM to the car after I bought it (the other Imps i looked at had XM but didn't have the other options I wanted). They could not deliver, so they bought me an aftermarket tuner and paid for the install and year of service. It was installed in the center console behind the shifter.
After about a year, a company out of Syracuse, NY developed a factory add-on XM kit for the 2006+. It functions 95% the same as OEM, only difference is that you don't get "categories" for some reason. The kit is about $200 and can be found on eBay.

