Ok. Do you have to plug your XM into the cigarette lighter to power it as well? If so take it to a custom shop and get the whole thing hard wired in, and see if they can wire the Xm directly into the system so it plays like regular radio, a cd or tape would. It gets to be a wired mess if you have all that out in the open and not hard wired.
If the car came XM capable the stereo might actually have an XM option in the default settings. That way you could just switch stereo mode into XM and boom yer playing crisp with no bs fm interference. If you can't get the thing hardwired to play directly, XM has a cassette tape that plugs into the unit and plays the radio through the tape. That's how I have mine set up and it works pretty well. It's not quite CD crisp but very close. I tried to play through the FM stations but it was awful sounding.