Just on a hunch, I went back and looked at the ECL (engine coolant level) schematic for the 1997 S320... guess what? The ECL circuit is entirely different and is tied directly to the washer tank level circuit... its here:
http://www.autolib.diakom.ru:8001/CAR/Mercedes-Benz/1997/S320/SYSTEM%20WIRING%20DIAGRAMS/6321.pdf
Also looks like its tied into the outside temp sensor.
Is it possible that although the car has a build date of 1996, it was an early "changeover" to the '97 electrical system under the hood?