Well, my 1930 Morgan as new, and even when bought by me in 1998 had no brake lights at all.
However, I eventually gave in, and fitted two brake lights. Being rear ended on a roundabout coloured the judgement although brake lights would not have been lit anyway as I was stationary.
The idea of fitting a third brake light has not gone away, but if I do, it'll be attached to the seat and very easily removable. The idea of making it a flashing one as above also appeals although it will be a LED flasher, as I don't have much spare electricity! Luckily, I can use LEDs legally. (Requirement for E marked bulbs does not exist for a 1930 car).
