You're going to need a wiring diagram. Find out where the power source is and the ground. If the fuses are OK, my guess is a corroded wire somewhere. Any other effected circuits? Older European vehicles can be a real nightmare because many of them have the same color wires for completely different circuits.
If you're 76 mite possible that the dimmer knob in the middle of the gauge cluster went bad. It's a series circuit I believe so it would make since that if it went the rest of the lights wouldn't burn. Just a thought.
The bulbs might be dead also anyway you have to pull the cluster to check anything. Also clean the big cannon plug's terminals by removing it.
All of the inst clusters on those cars are dim to begin with, but like said above the dimmer reostat is probably corroded.
Good thing about Benzomatics is they respond well to just being taken apart and cleaned.
My 300SD has 266,000 in it and will run forever, but shit falls off of it frequently, plus the underside of the car is so well lubricated from the engine and trans leaks it will never rust thru!
The OM617 5cyl is arguably one of the most reliable engines ever built. I have seen them in alot of other vehicles from moving vans to Haglund snowmobiles. The OM 616 (240D) motor is probably the most reliable of all. Only thing close is the 2 cylinder Onan generator motor, and they don't really count since they lead such a sheltered life. The Benz motors actually have to work. I have a friend who has a 240D with 800,000+ on it and he drives it 60+ miles to and from work everyday and has since 1982. Still will do 70 MPH on level ground too.
The instrument cluster on 70's Benzs pulls straight out the front far enough you can undo the speedo cable and oil pressure tube. Then you just jumper past the dimmer and have full brightness dash lights. I've got a few of those clusters in the basement because of odometer failures. If you actually need a working dimmer switch, post a phone number and I'll call you. Otherwise, there's always Adsit.com or 3Q's boneyard on Chicago's Southside for your parts for cheap.