As Shooter says, it's the rheostat. You might get lucky by twisting it thru it's full range. The ceramic glue falls apart and the spring gets messed up enough to not make contact with the wiper. I usually bypass them as I never dim the dashlights any way.

Michael

__________________
Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120