I talked to my local Ford assocessories expert and I wanted to do exactly what you want to do but due to a special wiring harness that Ford is developing, compatibility issue surround trying to do this. I cound not believe why this has to be so difficuilt. Then this expert along with a sales person pretty much trashed Ford's Nav system. I ended buying NUVI 660 refurbished from MegaGPS for 400 bucks. This lets me continue to use my Sync system and I bought a base mount for the GPS. This turned out to be the best way to go. The in dash NAV makes you look down too far. My NUVI 660 has a FM receiver to pick up traffic info and will re-route for accidents, road closures and slow traffic. E-mail me if you want a picture of how I have it mounted.
Good luck
Russ