OK now I found out what is happening after years of chassing.
Anyone else tell me what is 'wrong'
You have looked, after all these years and found the probem. You know the answer.
You would like for us, Americans, to give answers - Pass/fail.
I digress here a bit..
The American way of combining the rear indicator and brake light filliment together is crazy... well for non Americans as elsewhere these circuits are separate..but simple.
Which means wiring up a trailer to the rear lights in non American countries in such a maner ANY car can hitch up and have the trailer lights work corectly means running wiring right up under the dash.
If wired straight into the truck loom...when hooked up to a normal non American car, the indicator becomes the red brake light and the brake light becomes the orange indicator light...Which I have got around by choosing lens on my trailers that I can simply unscrew and turn over... If I remember... as my trailers are wired for towing with the Camaro
This is a hint, I would wager
Back to the issue above... why is my brake light no longer working on the re check?
Tail light power is its own circuit from the headlamp switch.
Brake light power is from the fuse panel and routed through the turn signal to each rear lightand to each front light; indicator on cluster is wired in with front bulb(s).
Directional light power is from a separate circuit/fuse to flasher then to turn signal switch. When turn is selected, brake light power is interrupted and t/sig power is introduced into the same brake light wiring to light the same bulb filament used for brake. Cancel turn, brake light power back into circuit.
Wire in a trailer plug, if dbl filament bulb, not a problem.
If separate bulb for turn AND brake on trailer, then a problem, maybe.
Both trailer bulbs will react the same, whether on a turn or brake.
Regardless of set-up, must change the flasher to a heavy duty unit as load has increased due to the extra bulbs from OE design.
Back to your problem, exercise all facets, and the circuits work - brake, turns, and back to brake. Get out of the car, close the door. Wait.
Open drvr door, press brake, one rear light. What is the problem?
Either one of two:
Lack of power going to the expected bulb - miswire, or too much load, for the circuit, not enough power being supplied due to too small wire gauge,
Pass door is not open to supply ground for missing brake light.