I've installed XM in my Audi allroad a few months ago, and am hooked. It runs through a cassette interface. I can buy an 'accessory kit' and use the unit in my CLK, but I'm wondering if anyone has mounted it. I don't believe the open space below the factory radio is large enough, so I might have to use a stalk in the passenger footwell as I did in my allroad. XM is pretty neat, particularly if you travel a lot, or live in an area with lousy radio stations, as I do.