I've been experiencing the same issue of the backup camera intermittently going to only black screen about once every 30 startups when in reverse but otherwise the nav/screen operated normally in all other gears (this is the key and only symptom, I had no other issues.) I had the dealership check it out and of course couldn't replicate the issue but foudn there's a TSB software update for the nav unit with this symptom, luckily I was still under warranty.
After they updated it I read up on the TSB and it didn't seem as if that would have been the only symptom of the software glitch. As expected that didn't fix it and it came back a month afterwards. However this time I wasn't in a rush to work and had my wife sit inside the car with it in reverse and parking brake on. I wiggled the sensor/switching wiring on the transmission (watch that hot exhaust) and smacked the gear selector and nav unit a few times for the hell of it. An older post on this mentioned the tailgate wiring breaking so I opened the tailgate and sure enough as soon as it unlatched and I lifted it the wife said the screen was working. Shut the tailgate again, screen goes black; open it up and screen back on. Had wife record everything happening on her cellphone for good measure.
Took it back to dealership and long story a little shorter they said one of the wiring pins in the harness had a loose connection and fixed it, sounds like its right at that rubber boot protecting the wiring going into the tailgate when it's open similar to that older post someone else mentioned. We'll see how it pans out going forward but it sounded more promising than the software update, though it sounds like a few of the users here might have better luck with that update from the sound of it. 2016 Trail as well with the Entune premium nav (not the JBL).
