I'm not sure man. I did a little "Googling" on the subject and didn't find anything useful.
I'm still thinking it's either in the antenna or an actual failed head unit.
My bet is that thin delicate wire that leads to the antenna base got pinched/cut/chaffed.
Most aftermarket installers will route those wires
under the rubber weatherstrip of a door to get the antenna outside. The problem with that is they've taken a thinly insulated 16 gauge (roughly) wire and bent it 180 degrees around two spot welded pieces of body panel and then pound the weatherstrip back on with their fist. The problem with that is the wires insulation got nicked on a sharp piece of spot weld. Plus, most of these installers choose to route the wire through the drivers door jamb......The door that's opened and slammed more than any other door on the car.

Or, they'll route it outside of the trunk.....The one door everybody seems to feel the need to slam the Hell out of to get closed. After a few months moisture has "had it's way" with the copper inside, thereby leading to poor reception. When the head unit can't "lock on", it simply gives up.
- If you installed it yourself, double check your routing and check every inch of wire for any signs of damage (focusing on where the wire exits the car).
- You had this done fairly recently didn't you? If memory serves, it wasn't that long ago. If a shop did it for you, it's probably still under warranty.
- Possibly a lose or dirty connection to the back of the head unit, but not the most common cause I've seen.
- I doubt it's an XM problem. I'm confident it's either reception of failed head unit.
Bob