Timbo,
Not sure if you have resolved this yet, but maybe I can help. I just added XM to my cobra Mach 460 as well.
As far as I know, Satellite radio wasn't actually integrated into any factory head units in 03 or 04. The 04 head unit has a 'SAT' button, but I think it actually had to be wired into an external satrad unit that would be mounted under a seat or somewhere.
The CD player in my 03 Cobra does not work, so I bought an 04 head unit from a GT (with a working cd) on ebay, thinking it would be interchangeable, but it wasn't. The Cobra M460 has a square 20 pin connector and the 04 GT has a rectangle connector with maybe 24 pins in it. So I would say to make sure the connectors will work before buying a different model year's head unit.
Anyway, since I have it in my f150 as well, but an external unit (an XpressRc), my intention all along was to wire it into the head unit with a PIE unit, like I had on my F150. That way I only had to pay one subscription, and I could just move the unit from vehicle to vehicle.
This link has a good picture of the back of a head unit that matches my 03 mach 460 exactly. It should match yours as well. Maybe it will save you from having to pull yours out to look.
http://www.tazcobra.com/460_mods.pdf
Also, here is a pic of my setup, if you are interested.
http://www.allfordmustangs.com/photopost/showphoto.php/photo/115483/ppuser/99455
I used a bracket from Pro.fit international to mount the unit. The wires from the PIE unit are ugly (and HUGE!) I know, but the sound difference, compared to a fm transmitter, is worth it. Good luck!