Check out the information I provided in the thread titled "Aftermarket Stereo" under the Technical & Modifications forum.

I did quite a bit of reading in these forums before I started out with the radio installation. Like you, I did not want to deface the beautiful interior with an add-on XM control unit. So the only choice was to replace the head unit with an XM-ready unit. From what I could read, the stock unit only puts out two channels and the amp and crossover do the work in driving the subs behind the seat. We connected my unit's front outputs only, and left the rear outputs unconnected. I think it sounds better than the stock unit, which I felt was disappointing. Of course, if you want a killer system, you're going to need to replace the stock amp and maybe also the speakers. That was just a little too much work (and expense) for me.

If you have any questions, you can PM me.