I spent some time on this today. I was sure that there had to be something amiss in the harness in the gate. I pulled all the trim.
Checked the grounds at the devices, result good.
Checked continuity through the rear stop light and license lights, checked good again.
Checked for power when the devices should have had power. Nothing.
Checked schematics again. These 2 devices only share a ground. A common ground to nearly everything in the rear door, and everything else was good.
Knowing that when everyting in a trouble tree checks out, before believeing it is some thing extraordinary, you should start all over.
I started at the beginning again. I had noted at one point that the license lights would light when brake was applied. With schematics in hand, this I knew now should be impossible. The only way that could happen is if it was mis-wired, or wires crossed or something. That should be impossible again.
I checked the wires running into the hatch again at the right top. This time I knew at least what color wires should be affected, so I gave them a good looking at, and I gave them a good tug.
And they were both in my hand! Success! I had looked at the bundle previously, and all looked okay, but buried in that pile of wires, those two were broken. I released most of the harness from the door, and I dropped the rear of the headliner, so I had enough slack to perform a repair.
I also noted rubbing/insulation breaks in some three other wires. I pulled them below the roof to have space to work.
I ran a few wraps of tape around them. Some people would try to just put a connector at the break, but I knew that they had been bent there many a time so I elected to install a layover harness instead, so fresh wire would be at the bend point. About 2-3 inches to either side of the break, I cut out the broken sections, in a staggered cut, so that at each side one wire end was longer than the other. This is so you don't have 2 connectors on top of each other. I installed a replacement piece of wire for each and used sealing connector on each side. In taking the harness apart, just below the affected area, there was a section of thick rubber plastic sheathing. I'd assume this should have been installed a little higher up in the harness, to help reduce abrasion, and was the root cause. I installed the sheathing in position right over where my other wear marks were.
I put it all together, and everything works awesome!
I'd have to hazard a guess that as mileage adds up, more and more vehicles could have this happen, so my advice would be to pop up that rubber boot, and take a look in there. If you see some wear and tear, it would be a good idea to work on it before it wears through and breaks, which will usually be when you need whatever goes out the most.