I've had similar situation on our 1997 C230 where the third brake light was out. I'd say it happens about twice a year. The first time, even though the old bulb did not look burned out, I replaced it with a new one. Same light out indicator a few months later. I removed the bulb, cleaned the electrical contact area, and put it back in. All was well. I think the electrical contact area gets dirty and trips the bulb out warning light. Good luck.

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230