I just bought a 1984 300D Turbodiesel back in Feb. My dash lights were so dim, I couldnt see my speed if a car was coming at me from the other direction with it's lights on! Although yours is probably not broken like mine, I noticed that the clear plastic panel that covers the cluster comes off (I think originally they were glued to the cluster housing). I noticed that the dash lights are actually bulbs that plug in the back side of the cluster but they don't provide "back lighting" instead, they somehow transmit the light up these clear plastic pieces that then have angled clear plastic faces on them that reflect back to the face of the instruments. I replaced the bulbs but also cleaned the clear plastic as the dust that somehow accumulated in there over the last 16 years had really dimmed the reflective light.
Hope this helps.
1984 300D Turbodiesel