Most probably a bad coolant level sensor.

If you have a later model W202 (I do not now the exact production date change but should be sometime around the 1997 model year), it could also be the washer fluid level sensor.

The two sensors are connected in series and have slightly different resistances (when the reservoirs are full). Measure the resistance of each sensor. The washer sensor should be 170 Ohms and the coolant sensor 110 Ohms. When either is bad, it will be shorted (0 Ohm).

You can clean the washer sensor. The coolant sensor can be removed without evening losing a drop of coolant because it is in a dry cone shaped chamber.

Again, what I have said is for later model W202s.