When you have a OSR brake light out issue on a 156, first checked the rear brake lamps, seem ok but wont work, buy new set, same issue. Get new bulb holder for the rear, same issue.
Now lest start getting technical, checking the loom and voltages, seem ok make sure that the wires are not broke during flex / movement....ok.
Still not working and the light on on the rev counter when the brake is applied.
Do a bit of research and find that my rev counter may be the issue, now not wanting to spend out £££££, checked the bulbs again and wobbled them about, no joy.
So I ended up opting for a rev counter in sheer desperation to have more than one brake light. Got on to a breakers yard and bought a new rev counter for the 156, same type and version as I have. Fitted it, and to my horror I had the same issue.
Multimeter out time.......check the voltages again on the rear OSR brake light, ......ok
Place the bulb back in the holder, bingo back on. Until I tried to clip it back in. All the plastic has melted around the fixings / come brittle with age, so the contact holder has come adrift. A Dremel later, and some small cable ties, I have stop lights.
Question is............was it the rev counter at fault or the holder causing the issue? got me confused, but to be honest it is all good again, so I will leave it there until it throws up another wobbler.
On a bright side, if I do get an issue with my rev counter, I have a spare