On my 92' 300SE, there's a coil in the tank that has a thermostatic valve in it - I think it only opens when the temp of the washer fluid is below a certain level. The lines from the tank to the nozzles have a heater wire inside. This is also thermostatically controlled. The nozzles themselves are also heated, as is the base of the windshield (to prevent the wipers from sticking).

I don't know if this is an S-Class only thing. Knowing MB and the belief that the build cars to work anywhere in the world - possibly most all models have one or more of these features.