Strictly speaking, the third brake light has been mandatory in Germany since 1998... at least for cars registered after that date. Older cars do not need to be retrofitted with a third brake light.
I haven't had any problems with the Morgan yet. Our case blocks the view of the third brake light. With a Morgan, people don't usually check the year of manufacture meticulously. But strictly speaking, if it's there, it has to be used. It's a borderline case if it's there but obscured.
Perhaps it depends on the country. I have encountered very strict police officers in France. We were filling up on the highway. A police officer noticed that there was a little water in my Morgan's front turn signal due to the pouring rain. He ordered me to buy a screwdriver at the gas station, unscrew the turn signal glass, and pour out the water.
