Like 250Coupe said, installing some extra grounding wires might help. Try hooking up a heavy guage wire from instrument cluster housing, say from one of the 7or8mm instrument attachment nuts to something on the firewall. And in the engine compartment you can hook up a massive braided grounding strap from engine to firewall, say from a bell housing bolt to something that attaches to sheet metal. Sometimes there's one there already. Also hosing electrics with WD-40 can help. And so can unplugging and reattaching electrical connections.

The best remedy I've heard for curing 19th century technology of the instrument light switch is to bypass it entirely. That dimmer switch is an inside joke among MB owners, easily the most primitive component on the car. Earlier ones are white porcelain and brass, very impressive looking and resemble something you might see at the Thomas Edison electrics museum.