I was replacing the festoon bulbs in the light fixtures on my 28, these are the original fixtures, when I replaced one of the incandescent ones with an LED the new build lit up without the light being on, it was not very bright but was most definitely on.
I took a close look at the switch contacts and they were not touching so clearly there was another path in the fixture that was allowing current to flow through. I cleaned the fixture, dried it and don't it back together and the fixture seems to be operated correctly now.
This must have been drawing a small amount of power for some time now, not enough to light the incandescent bulb but a power draw none the less.
I can certainly appreciate the simplicity of those original fixtures, not a lot to them and they can be easily fixed.
New lights work well, not bad for 0.79$ a bulb.
Sawlty