I did fit an extra earth cable to muy WC54, from a BMW of all things! It was there and had connection eyes in all the right palces.
Though I've had 12 volt earth problems as well. My WC51 once drove out of her shed, I switched her off and she then refused to do anything. Problem was the main earth strap, it looked clean and was tight, but being unable to think of anything else, I took it off cleaned and reattached, away she went as usual. :nut: