retroing factory nav on 05 expedition

I bought a 05 Expedition Limited edition 4 months ago and I want to add nav/btooth/xm/ipod capability.I"m getting conflicting info. One dealer says yes and the next says no. Same with the installers at best buy and circuit city. The forum on here had a guy who retrod an 06 factory  nav unit to an 05 F150 . Can that work on my EXPI??? Whats the big deal with the expedition?  I refuse to accept that its not possible. An after market unit would be great except I was told it would have to be relocated under the seat or in the console somewhere else to keep the rear overhead unit  at a cost of 600. to 1000. extra on top of the regular for the new unit and install.Thats absurd!!  If this is true and the factory cant be added,,,, nice job FORD. This will be the first and last FORD I buy!