Similar problem on my 98 C230. Indicator showed a bad bulb and I did see a tail light was out. However, the bulb was good. It turned out the little metal "finger" contact in the bottom of the socket was not making good contact. Also the little solder bump on the bottom of the lamp was worn a bit. I cleaned the contact and bent it up a bit and solved the problem for now.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra