Antennas
I'm assuming you are using some kind of inside the window mount.
Finding a place to mount any antenna would seem a tough job. I guess thats why we have that poor windshield job for the radio. Anyway given that's where it is, I doubt the window has the kind of metallic content that would block signals.
As an alternative, arn't there XM antennas that mount outside the cab using magnets?
Edit: Try searching
www.xmfan.com forums for info about antennas.
I think if I were looking to mount an XM antenna, I'd try one of the mouse mag. base types near the back edge of the tonnue cover. Then look for a way to snake the wire through the top's bucket.