Instead of replacing the factory radio, why not just ADD an XM modulator?
Don't know about Sirius, but XM has the Roady2 on sale for $29. Comes with everything you need. Activate it online and avoid the $14.99 activation fee. Pay for a three year contract and it works out to $9.99/month.
The Roady2 comes with an FM modulator, which plays the satellite channels through a selectable FM channel. It sounds fine, but there is a slight degredation in sound quality. If you're really concerned about the fidelity of the music, you can always add an inline modulator that plugs into your FM antenna cable which will produce great sounds!
www.xmradio.com