Indeed. Looking at your wahser reservoir, you will see one or two pumps (depending on if it is an estate or not) and a large cap with two wires attached. This is the sensor for the fluid level. Remove the cap and pull out the sensor and what you will find is this: inside a tube, there is a float running up and down a pole. At the bottom of the tube, there is a small hole. With the reservoir full, the water goes into the tube and pushes the float up, thus breaking the circuit and no light. As the level goes down, the float sinks to eventually close the circuit and switch the light on. This float sometimes sticks due to gunk so the first thing to do is to try cleaning it out with some clean water and simply shaking it until it can move freely up and down. If that does not work, I suggest getting a new one, they do not cost much.

George
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1987 745 GLE Turbodiesel Intercooler (D24TIC/M46) 190k and 1988 745 Turbo Intercooler (B230FT/M46) 290k