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I added XM to my Jeep and to my 2003 Vette. Easy to install, but a few warnings.
One, if you plan on doing any off-roading with a GPS unit near the XM receiver, do not purchase a Roady (there are some new ones out there still). Roady 2, may cause problems with some GPS units, but not with Magellan.
The Roady interference with GPS was confirmed with a call from Delco, and my own trials.
I also recommend a unit you can actually connect the unit into the antenna wire. This prevents you from having to switch stations when traveling. I traveled from MI to Utah in my Jeep and never had to switch frequencies on the radio. Also, I hard wired both units into the power/ground too.
I ended up with an Audiovox in the Vette, and the screen is totally unreadable in daylight due to the blue screen, and the buttons are so small it is next to impossible to switch without holding the unit.
In the Jeep I installed a SCIFI II, screen is readable even with the top off, controls are easier to use (note larger buttons), and I also take it into the house and attach to the stereo with the home mount kit.
I would opt for the SCIFI II, but that is personal opinion.