I assume from your post you are using FM Modulation.
With the new XM and Sirius receivers the FM Mod stinks, the problem isn't that you don't have a clear FM channel to listen through, the problem is because of a FCC ruling a couple of years ago the FM Mod signal in the newer XM and Sirius units is weak,so you will hear a lot of static and interference on the FM station.
You need to connect you XM receiver to your radio some other way...Aux-in? Cassette adapter? If you don't have either one of them try something called 'XM SureConnect', it's like a direct FM Modulation. Instead of using a weak wireless FM Mod signal it connects directly to your car antenna and uses a direct, hardwire signal..If you look around the Net you should be able to find it for about $15...And it is easy to hook-up and works good. Make sure it's compatible with your radio.. Looks like it is..
http://store.xmfanstore.com/sureconnect.html
http://www.xmradio.com/sureconnect/index.xmc