I don't know about this particular model for sure. I expect that it is a potentiometer like the 123 cars. You can probably push out the cluster and solder a bridge across the potentiometer to give full brightness. Make sure you bridge the correct terminals.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles