L.A.B. and Rohan, thanks for your replies.
The bulb holders do not have ground / earth wires, the instruments are mounted using two studs, the studs have a small rubber ring sandwiched between the back of the instrument case and the mounting plate which bridges the forks. The rubber rings were original pieces ... or so I'm told ....I have not been able to find them in parts manual
Removing the rubber rings will remove any vibration dampening to the instruments, perhaps adding a ground / earth wire is the solution ?.
Thanks again for your comments.
L.A.B. my multimeter has a " continuity " setting, emitting a " tone " at continuity.