I just installed an XM Roady2 in my MS6. Placed the antenna right next to the FM antenna above the rear glass. I fabbed a metal bracket and secured the Roady2 mount to the ashtray. The radio sits right above the ashtray, so it's easy to see, and easy to push the buttons while resting your palm on the shifter. I placed an auxiliary 12v socket (Radio Shack #270-1556) under the shifter console, where it's tapped into the cigarette lighter wires. Then, the Roady's 6v adapter plugs right in, and everything is hidden, out of the way.
I've already ordered the RPM audio link so I can get some decent sound input.
Here's a pic:
See more pics
here.