All external bulbs including stop and reverse. If you are sure all your bulbs work the relay is under the screwed down cover behind the fuses and is the longest relay in there. The cooling fins for the headlights can go out and a new relay is needed. If it halps it is the same relay in the 126's. I got mine from a newer 95 MB in a bone yard for cheap and been good ever since.

Also wrong wattage bulbs will draw too much and make it come on. IE fog lights are 50-60 watt if you get the brighter 100-110 ones it will come on when you turn on the fog lights. Many threads on the wattage and the bulb out indicator relay on benzworld.