I've had 2 'interesting' rear lighting malfunctions over the years.
One car had a report of a LH brake light out. When I checked it, it was always fine. A nearby motorist would tell me about it about 1/month. It would only happen if above ~60° and if it wasn't raining.
It turned out to be a mildly-weak TS cancelling cam, it would hold the circuit open for that light. I believe the weight of the stalk allowed the RH to cancel OK, but not the LH. I was able to fiddle with it and find the dead spot, to literally turn the problem on/off.
Any ideas what the temperature and rain had to do with it?
Car #2 (this may interest you
@Chris N ):
#2 was a 70 Sport Fury (the Great Pumpkin of GT#3 fame, for those who remember that) that had a rear TS that wouldn't blink properly.
It ended up being weak grounds.
The lightbulb sockets at the rear are swaged into the housing, and apparently that ground path got weak. I removed the lenses and went around the edge of that swage with a spring-loaded center punch and that was a simple fix. But it didn't last long enough.
The permanent fix was to sand a clean spot on the socket (behind the bumper), smear some anti-corrosion grease, wrap some wire around and affix it with a hose clamp. The other end of the wire went to a good ground nearby on the car. That was a permanent fix for as long as I had the car.
I don't know for certain, but I suspect that swaged-socket design might be a Fusey-wide thing.