I just had XM put in my Macintosh equiped 2005 GT - I can tell you that it was a royal pain-in-the-ass but these guys at Speedometer Serive Co in Roswell, GA (770) 569 8255 know their stuff - They are the only factory authorized shop in Atlanta - They very carefully took out the radio, the center console and the backboards in order to run the unit from the back of the head unit to the front of my seat.
They put the XM To Go, unit and wires, in the front mesh of the drivers side front seat so all I have to do is reach between my legs (no, not an old Van Halen song) and ease the seat back...........and "poof" the unit is working perfectly!!
The antenna actually got mounted with double side tape in the front of the dashboard/windsheild (right on top) and it looks factory and works like a dream!
The car went back together with not a scratch or a nick and no rattles!
The only downside is the aux out from the XM to go plays lower into the unit then radio and, thus, you have to turn it up to hear it - so just remember that when you turn it off and it blasts your ears because you forgot to turn it down!!
SGB