After replacing instrument panel bulbs and potentiometer on my 1977 300D/123.130, the lights were still "too dim". I undertook the following effort and it worked,,,,,,, for once.

Remove the instrument panel. Remove all of the guages and clean them and clean the inside. I just used H2O and paper towels.

I also cleaned the plastic "prizm" 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 desparation, 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.

JCD