I have researched the oem units quite a bit, I really do prefer the factory look to after market plus the factory units are plug and play, remove the old unit and plug the Nav unit in put in the 4 screws, in a snap your done.. There are units on ebay that can be bought that will fit the 05 through 08 Explorer / Sport Trac for anywhere between $500 and $1100 then they can be programmed for another $80. to $100. There are a couple dealers that will do the programming when you buy from them. They are 6 disc and will work with the Sirius, steering wheel controls and most also include the antenna and 6P or 7P disc.
However if you have Sync there are far fewer units that will work and In addition I am being told that you have to do a button swap and some rewiring to make the steering wheel controls work on the Sync, of course this does cost extra. I have been trying to find a picture of an Explorer or Sport Trac steering wheel control that came with both Nav and Sync to see if the button arrangement is indeed different. So if anyone does have factory Nav and Sync please post a picture of your controls