I know in my X (03, 3.0) The previous owner installed an aftermarket sirius radio that was FM modulated. When I removed it to install my XM (which I love!!!) there was cables for the FM antenna. I don't know if it was an extension added by the installer of the sirius radio or if its there. It was located behind the radio. I had to pull everything out and dig it from behind. I would not have found it if the sirius radio was not plugged into it. Maybe that will help you out.

I agree with previous posts though, sell what you have now and spend the extra money on the dice kit. The X stereo isn't bad, but you can really find its shortfalls when listening to anything over the FM or AM bands. Just my 2 pennies :P

Good luck! Let us know what happens incase someone else finds themselves in your situation.