I assume you got the Pioneer antenna?
I used the Terk SR1 Glass mount http://www.terk.com/ because I didn't want to drill or run the wires over the trunk gasket, none of the XM antenna are very pretty
On the back glass or the roof is the preferred mounting location, all the manufactures recommend having the top of the antenna higher than the car for uninterrupted service, but............. I have talked to several people that have had no problems with mounting the antenna INSIDE the glass such as on the rear deck, as long as you have a sloped glass and the antenna can "see" the satellites.
In my own "limited" experience with XM...... I haven't driven the car yet with it installed however it has been activated for several weeks and even setting in my garage with an attic full of junk overhead I have no problem with signal and have had uninterrupted service.
if your radio is xm compatible do you need any more equipment besides a antenna? ballimpala how do you like that 8400 i saw it in the store but my pockets weren't that deep to get it so i settled for the 6400