Going from memory here, but I believe there is a central ground on the firewall behind the instrument cluster. Believe there is another on the passenger front sidewall by the OVP relay. One more I think, but don't recall where.
Also, I have an instrument cluster I bought off eBay that I can part with reasonably if you must have one.
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000