I have several friends and coworkers that have XM equipped GM products with Sirius. Most of them just place the antenna on the dash board. If you feel uncomfortable running the antenna cable under you headliner or under the kick boards, consider a professional installer at Best, Circuit City, Tweeters.....etc to install for you.
Personally, I have a Chevy Trailblazer with Sirius and I use the SIGMA (Sirius Interior Glass Mounted Antenna). I mouted it behind and above the rear view mirror. Works great and this is the same setup that Toyota and Nissan use for Sirius.