Rear light on vintage motorcycle 6 V split to 5 V
Just fit an easily removed deceleration lamp and save all the faffing about: -

Our rear-end collision avoidance system is simple yet effective. After
you install our deceleration light on the back of your truck or
trailer, it will light up and flash when you start to slow or brake.

Here's one from Shamazon: -

And here's another one that can be fitted to motorbikes: -


The problem I have I don't want to destroy the appearance and remove
the old light or fit light boards etc and want to keep it
aesthetically pleasing for the rivet counters.

You can even get ones that fit to helmets thus pleasing the rivet counters (if they could ever be pleased!): -