I tried the approach below on my 1977 300D. This was after replacing the bulbs and the potentiometer. This is either a creative or "you must be nuts" approach, but it worked. Not sure if you have the same set-up, but if you hate dim instrument lights as much as I do give it a try.
Remove the instrument panel. Remove all of the gauges and clean them and clean the inside. I just used H2O and paper towels.
I also cleaned the plastic "prism" that directs the light from the bulbs to the light diffusers that are in the front, nearest the driver, of the instrument panel. Again this was with H2O.
I noticed that the diffusers were a dull "yellow". My sense that the plastic had weathered or degraded. They were "welded in" via heated plastic tabs so replacement appears out of the question. In desperation, I applied some white flat paint to the diffusers, the idea being a brighter surface.
It worked. I took a steady hand and a small brush, but the instrument panel lights are much brighter.