I just installed an Antenna Specialists XM20/CP antenna in my Coupe. I installed it under the brake lite housing on the rear deck. It fits fine. No holes to drill either. To access the housing, simply lift the housing and pull toward front of the car. Slip the XM antenna in, and run the wires through the hole forward of the trunk light. You will likely have to remove some of the padding in the brake light housing. Then re-install. Works fine, and no ugly antenna to see anywhere. Anyone can install this antenna.
http://www.sdarsantennas.com/prod_552.php