You're probably better off having this done aftermarket at this point. From a cost perspective it's probably cheaper to purchase a receiver and have an car audio shop integrate it in to the factory radio.
Dealer claimed the can add SAT radio to the base stereo, around 400.00 was what I was quoted.
I ended up just buying one of those XM snap radios for 80.00, I leave it in the console compartment and it goes through the AUX input. Took about 10 minutes to run the wire for the antenna, I just double sided taped it to the bottom of the rear window. Works great, have to open the little door to change channels but its not a big deal.
id also like to do this. $400 is nuts however. What kit did you go with? Audio stuff is another world to me. A friend of mine who is more into it told me how much i got screwed when i added a sub and amp to my car. Worst part was when i googled the parts I got and saw the prices theyre going for. Lesson learned lol.