It is supposed to be set up to add on an external satellite unit. On one of the Mustang sites (Brad's ??) a photo was posted showing the setup. The satellite unit sat in a cradle and was removeable so you could bring it with you.
But, I'm waiting for Ford to offer a completely built in Sirius unit with, AM / FM / CD changer.
Why would you want to keep the stock deck? I might keep the 10 inch sub if its any good but I doubt it will be better than the two MTX 12 inchers I got. The first thing to go will be that deck. Got a brand new Alpine with the 7 inch touch screen all ready to go