There is a bulb behind each LCD display that will dim as soon as you turn on the lights (depending on how the dimmer is set). IF your LCD displays are not visible at all or do not react on the lights being turned on/off, it is possible that you have a worn bulb.
Start with adjusting the dimmer to the absolute maximum of light.

greetingz,

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