I have a fully functional 3rd high mount stop lamp.
I have changed the fuse in the trunk for the stop lamps.
I have replaced all the stop lamps.
I have tried swapping relays
I have sprayed down the contacts with contact cleaner on both the relay and the shunt. That worked for a day, and then the problem became intermittent, and now it is back full-time.

Since the spraying down of the shunt, the relay and their sockets seems to work for a short while, that should be telling me something. If i go back and just remove/replace the relay, that seems to get it back to intermittent again, but after a few brakings it just returns to non-working status.
any ideas? Is there a better way to clean those contacts?

 

I saw a post somewhere on this forum where the author diagnosed a loose connection on the relay board where the brake light relay went. My symptoms were: cleaning the terminals definitely had a temporary effect, but the dreaded "check stop lamp" message soon reappeared, and it did so trying several different relays and shunt resistors.
His "fix" was ( and at 166k, I'm not above this) to wedge a piece of folded cardboard between the offending relay and the adjacent one, thus wedging the contacts tighter in thier slots. IT WORKED. FLAWLESSLY. NO MORE FAULTS.
Oh, I'm aware that this would not do for a "professional" fix. Perhaps there is some way to retension those contacts inside the relay board, or replace it altogether. But the cardboard wedge accomplishes the same thing. It simply angles the entry of the spade connectors to tighten their contact.