My '85 300CD had the "dim light" problem assoiated with older MB's. When changing out all the dash bulbs (might as well do them all while in there), my mechanic suggested by-passing the dimmer switch...claimed it would help a little. As I never dim the dash lights, I let him do that.
Don't know if it really helped but my dash lights are sure better than they were. Still not what I'd like.
Hard to believe that this nice car has such a bad dash lighting system.