I used the PAC-Audio - SWI-JACK interface that is a programmable adapter, together with a Kenwood KDC-BT60U (FM/CD/BT/AUX/USB), and that worked perfectly... The same adapter can then be reprogrammed to whatever reciever I'll switch to next. Happy times!
I wrote a DIY article on this topic for the Norwegian XM Club, but as it is in norwegian it might be less useful

The guys at PAC-Audio was very helpful though, so I doubt there will be any problems figuring it out

You'll have to modify the normal setup a little bit as the XM S2 uses two "resistance ladders" for the steering wheel controls and the SWI-JACK is made for one "ladder" ... adding a (I used a 420Ohm) resistor in series with one of the ladders before connecting the two in parallel did the trick for me...
PM me and I can send you diagrams and stuff

anders