I have Sirius Plug and Play. Bought a bracket at Best Buy and my sound guy drilled the bracket inside the cubby hole below the Porsche head unit.
I also gave my sound guy a Becker auxiliary jack that plugs into the back of the
Porsche radio so that you don't have to run your MP3/XM through an FM requency.
You'll have to use the cigarette lighter to power the XM plug and play unit.
The ideal set up is to buy a head unit with XM built in but i think you'll get charged for two subscriptions if you decide to buy an XM receiver for your home. In terms of sound quality This is the best way to go. No chance of getting alternator whine that way. I use a SOny 12V cigarette lighter adapter to power my Sirius which reduces the whine significantly.