As a follow up, I just went out and looked at the antenna closely and realized its actually very easy to remove.
If you pull down about 12" to either side of the headliner, right up against the back window, there is a single round plastic snap on each side that pops right out.
That gives you enough room to look underneath the headliner where you will see two cables plugged into the antenna, a smaller one for the XM and a larger one for the GPS. Those cables are push-in, so all you have to do is very carefully pull them straight out. After that, there is a single nut, about 16mm that holds the antenna in place. Mine was so loose, I could turn it with my fingers.
I took mine off then examined it before disassembly, where I could see why it failed. There is a rubber gasket that was not properly installed at the factory that was allowing water to get inside the assembly.
The bottom of the antenna has five Phillips head screws that hold on the plastic "Shark Fin"
Removing those reveals two circuit boards that have the actual antennas mounted to them, both held in place by six Phillips Head screws.
I removed the board for the XM antenna, very carefully prying it straight up to remove the Teflon insulator from the middle of the antenna connector and it revealed a pocket where water had sept in under the gasket and was shorting out the board.
I dried everything off, reassembled it (putting the gasket in its proper place) and now it is working again.
I wouldn't recommend doing this unless your car is out of warranty and you are comfortable with this kind of stuff.
Also, make sure your hands are scrubbed clean before you touch that headliner...
If anyone is interested, I took photos of most everything and I can post them on Monday...
Tim