Okay, so I had all the trim off of the hatch, and no luck with finding any kind of wiring fault.
I still haven't pulled everything up front at the APIM, so the jury is still out if it's a "simple" fix of something in the wiring.
I did get the rear camera for the dashcam installed. I just ran the cable from the dashcam to the back along the edge of the headliner on the passenger side. easy enough to tuck under the edges at the front, under the edges of the A and B pillar trim/airbags and along the tops of the door seals under the headliner. back over the C pillar trim and to the wiring harness to the hatch.
FWIW, I did have to pull the headliner down a little to get to that, and the wiring harness there, but I didn't have to pull it down much.
I installed my dashcam to the trim piece at the top of the hatch, so that it peeks out just below the top of the arc where the rear wiper cleans the glass. the dashcam also has a feature where if you connect a wire to the positive wire for the reverse light, it can act as a backup camera, though with it mounted at the top of the window like that, it isn't the best backup camera. I did connect it though. I tapped into the green/brown wire at the rear light harness, which I determined to be correct. Since it works as expected, I guess I was right. :)
I still want the backup camera working. I don't think that the camera itself has failed. I suspect that it is a wiring/connection issue. I hope that it isn't a problem internal to the APIM. I don't believe that it's directly related to the display, since that works properly for the entertainment/navigation system, and shows the "camera unavailable" message.
I did get a close look at the lens, and it's scratched up quite a bit, so I believe that this explains the poor image quality on the display. I expect that a good cleaning and polish will take care of the problem, assuming that I can get it working.
2012 Focus Titanium Hatchback
121,000 km, all stock
1990 Toyota Supra turbo
I think the cams are still stock...