Morris Minor Owners Club

Dodgy earth??

check the connections on the black wires and where they are / it is connected to the body.

Yep, defo dodgy earth or a short somewhere. Most likely the former. Undo the earth wire that's attached to the body as Ray says, give it a clean up, clean where it's bolted to & the bolt & try again....

Definitely a poor earth unless this also happens when,say,you
turn on the headlights or the windscreen wipers?? In which case
you have a battery problem.