I bridged my rheostat connections and it didn't make a difference in my case. These units were NEVER very bright on the W123 and early W126 when new. Don't put in brighter bulbs, as they will overheat and place a brown film on the clear plastic prism that carries the light from the back of the unit to the front. I repainted the area where the bulb is housed with white paint. This helped a bit. On my 1981 300SD, I ended up buying a new instrument cluster (minus the instruments of course) and this greatly improved it. This was 15 years ago! Not sure if this part would even be available for the W123 (and the cost may be ridiculous!!).

__________________
All the best,

J. P. Mose
1968 250SL
1970 280SE 3.5 Cabrio
1987 560SL
1990 560SEL