Assuming you're talking about the lamp-out indicator, not the charging system light, right?

Check license plate lamps.

Also - sensor is very picky about bulb wattage. Make sure that all bulbs in rear lamps are of correct size. On my '92 300SE, the size (MB specifications) are visible next to each socket. May also be available in the owners manual. Even one bulb of the wrong size in a socket is enough to trip the lamp out light.

Diagnose which bulb by when the indicator comes on. If it comes on when you switch on the parking lamps, then it's one of the parking lamp bulbs or the license plate lamps. If it comes on when you step on the brakes, then it's one of the brake lamp bulbs, or the high-mount stoplight.