Ducati electrical weirdness

[Speed addicted]

My Ducati ST2 project bike has an electrical issue, when the lights are off and you use either brake it lights up the clocks and the sidelight.

If the lights are on it all works normally.

I’ve fitted an LED rear light, and it looks like an earth fault to me but everything’s earthed properly.
I put additional resistance in using one of the flasher resistors (for fitting LED indicators) with no effect.

Any ideas on fixes, or where to start looking would be handy.
It will be ready to MOT once I get a functioning exhaust sorted out so this is pretty annoying!