I hooked up my XM MyFi (the portable) in a matter of minutes using the FM modulator sold for that unit. Plugged rright into the Subaru unit (6-disc).
These companies may sell what you need:
http://www.scosche.com/
http://www.everythingipod.com
http://www.macmall.com
http://www.belkin.com
http://www.densionusa.com
I could go on and on...
Also, search the audio/video forum here and on IcLub and you'll find a considerable amount of discussion about hooking up the iPOD