Wel, I've answered my own question... Poking around the forums here, I realized that antenna is standard for many GM vehicles. So after googling around for a few minutes, I came across this page:
http://www.archivedsites.com/techlink/2 ... tenna.html" onclick="window.open(this.href);return false;
Basically you just have to press down on that little tab/slot thingy I mentioned with a small screwdriver, and the XM antenna twists right out, easy as could be. I'm very happy that I won't have to mess with dropping the headliner or anything.
Thanks for chiming in, trb!