Nevermind the body type you've got, the rear socket is not listed in any of the wiring diagrams I saw (van, panorama, cabin crew, minibus). So in fact getting the correct body type won't help
In the beginning I assumed that it is a dealer / converter installed socket, but I found the same fusebox / relay diagram + description you and MSjoberg have seen and it clearly says the T17 relay is for the rear power socket. That's why I was complaining about the Italians.
Hope it will get the power once you put the relay in place. If not, it will be difficult to trace the installation, as it is not described in the available eLearn. Good luck
Did the same on my van, I have the ingition powered reverse camera
But it was not factory installed, so I just run a wire on my own.