Thanks for the comments. Although the car's a 1996 model it seems to have simple reed switches in both the w/s washer sensor and the coolant level sensor. (However both the od and replacement coolant level sensors flips between 0 and 500ohms with level changes whereas the w/s washer sensor flips between 0 and open circuit. )

I've also noticed that one of the wires to each sensor has the same colour & band.

Does this mean they're wired in series and there's a circuit somewhere that looks at the total resistance and works out which light to illuminate?

Cool site BTW. Shall pass url on to other MB owners.