TLDR: camera feed leads labels were switched.
Finally got round to installing on my Ducati Multistrada, replacing the factory fitted SatNav, which I've never got on with. Found the installation fairly problem free except for one issue: the camera feed leads were labelled incorrectly. The lead labelled Front actually carried the feed for the rear camera display, and vice versa. Unfortunately, this can not be changed in the settings, so I had to remove fairings to get to a point where I could switch the leads. Moral: check before fixing the wiring and replacing fairings, and check more than "yes, there's a valid input feed".