I posted earlier trying to find out how XM could be installed in the Thunderbird without a satellite dish disquised as a contenintal kit. I know they make fairly small antennas for XM, but they are still bigger than a cell phone window mount.
There is a window mount antenna (very ugly) available. I have an RF received that plugs into the radio antenna input. The module is hidden somewhere under the dash. Since it's in a Chevy Tahoe, who cares what it looks like, but I wouldn't want it on my Bird.
If you find a section somewhere on the top of the car that is metal, you could have a conformal array antenna fabricated. I'm currently doing this using an aircraft style GPS Discrete receiver and Palm IV with Voice Recognition-the antenna will be part of the roof of my 93 LX