I just purchased a 2011 Expedition XLT 4WD with leather, but no nav either. I also wanted to add in the nav. I was going to buy from ebay and have dealer install. I contacted my dealer, and the service manager said that he did not know and would get back to me. Today, he called to say that they contacted Ford, and Ford does not recommend it. I don't know how truthful this is. From my reading of previous posts, all I need is a nav that will work for my model year and then have it "programmed" with my VIN so that the nav doesn't think it is a stolen unit.
anyhow, this is too much trouble, considering my dealer doesn't even want me to pay them to do the install, which I suspect should be plug and play after the "reprogramming". I just purchased a Kenwood DNX 690 (2013 model) and the Maestro adapter at Crutchfield and there is a special rebate for the Sirius tuner for free. This should preserve the steering wheel control, ford sync, and the Sirius tuner would work (only 2013 Kenwood lineup only).