W123 300D,

The switches on the center console are illuminated by one bulb that is placed behind the console panel. This bulb is attached to several fiber-optic cables to illuminate the switches. If the bulb is worn, all the switch-lights go out; you have to replace that (one central) bulb to enlighten the switches again.
The bulb is stuck into a fitting against the heater-housing behind the center console panel.

The bulbs in the dash cluster of a W123 can get very black and the clear plastic parts inside the cluster that reflect the light are occasionally melted (because of higher Watt-ed bulbs).
If adjusting the light intensity does not help anymore, exchange the bulbs and have a look in the cluster (through the light bulb openings) to see how the clear plastic diffuser looks like.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)