My 1979 300SD used to have turn signal, flasher, and tachometer problems. I pulled the instrument cluster out to find some of the wire connections for the bulbs broken. The broken wires would short out on other things. Sometimes when I used the turn signal it would make the tachometer jump around. The bulb wire connections behind the dash cluster are very fragile and can fatigue. Maybe try inspecting behind your instrument cluster if you haven't already.