There is no aux input to the factory head unit. I just went with Sirius over XM. Needless to say I went the Modulator route (Kenwood). The quality to me is no better than the standard radio.
The big difference with XM and Sirius is XM is Mono, Sirius is Stereo and has a lot better coverage. Programming is apples to apples. Sirius is also $2 more a month. No matter what you read XM and Sirius are in big financial trouble so beware. Check out the following.
http://www.usatoday.com/tech/techinv...te-radio_x.htm