I saw your other post about rear view mirror cam. I'd suggest you just go with an aftermarket solution for GPS and backup camera.
Watch this video if you are still interested in adding the OEM Nav unit. In addition to the above mentioned VIN programming that will be required, you'll need additional hardware and cables as shown. Its not a plug & play swap unfortunately. Factors about your Navigator having the Sony or THX system or not, sub or no sub, rear seat entertainment or not, SiriusXM tuner or not, and who knows what else will all come into play. It even looks like the steering wheel controls will likely need to be changed out. Seems like a lot of trouble for a nice system when nicer can be had for less $$$ and effort.