In a 2013, remove the a/c & vent knobs by pulling them off. Using your fingers, pry off that panel. That will expose the four 10mm bolts that hold the radio. Remove them & pull the assembly out with your hands.
The XM antenna certainly can be roof mounted, and works best that way, but is more of a challenge to route the wire that way.
The XM reciever fits nicely under either front seat, and can be secured with Velcro.