I was almost ready to run a ground wire, but no....

I struggled to get the rear deck loose and finally after it was out, found that the 12V was fine at the harness connector to the deck light.
What isn't obvious is that there's two assemblies that come out, and another screwed down to the deck
First you press in the square buttons on both sides of the outer cover and pull
straight to the front.
Then is the bulb socket and reflector, that has a plastic latch lever on the side to release and remove to pop in a new bulb.

With that out you see the wires going to some spade lugs and I had no ground.
Then there's an almost invisible plastic latch lever in the center bottom
that you bend up slightly to release the second assembly where the trouble was.

The second assembly has a pair of spring contacts that mate with the final base
assembly which is screwed down to the deck with two Torx head screws and the rear
window makes it an adventure to remove. But you don't need to move it.

I just formed the end of the contact that was broken and put a dab of dielectric silicone on the contacts and bingo.

I looked at the light at the junkyard and it had a snapped off contact too.

I didn't need to mess with the deck trim at all, and it turned out to be a
trivial repair, after you see it apart.

The second or middle assembly has some tracks to replace and it moves only
straight forward and straight back.

I hope this saves someone a headache, as it was the solution to an at first
intermittent, and then dead high brake light.

Bill