Thanks all that have replied...

I browsed the CD manual last night, it indicates that for post 8/85 126 models, there's a small bulb behind that orange plastic dot at least; but I couldn't find the instructions as how to remove the knob to investigate. Don't really know if there are other bulbs for the switch plate though.

Frank.

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'86 560SEL | Silver Ext. / Blue Int. | 201K miles (Apr. 05) | 204K miles (Feb. 06)