I've installed aftermarket rear-view cameras into aftermarket head units. Pretty much plug and play, the toughest part is mounting the camera and running the wire under the carpet all the way up to the head unit....
They work off the TV standards, so they are compatible....
Then you have to go into head units menu and enable and configure the camera, not too difficult and you're not sure how to configure it, its simple enough you can experiment with the settings till you get the best picture....
But you want to add it to an OEM radio (even though the guy had the Dealer install an upgraded radio, its still an OEM radio).... ....the bad with that is likely the Head Unit Menu is all done through the Dealership tools....
I've found the OEM rear veiw cams on ebay for a reaonsable cost, not sure how the post 2014 fit the camera in the rear deck lift, the earlier it was in the plastic trim panel over the license plate... ...but chances are likely the wiring is already in the vehicle, its just a matter of plug and playing the camera, except for the configuration part....
Perhaps the FCADiags or AlfaOBD could configure the head unit for you at a lower cost than taking it to the dealer?
Sorry, not the specific answer you're looking for, but hopefully it moves you along in finding the full answer....