2002 E320 Windshield Washer Reservoir Heater

[TomLatos]

Can this expert community let me know if the Windshield Washer reservoir heater on this car is thermostatically controlled?

If so, what are the conditions that would enable engine coolant to flow to the coil in the washer reservoir.

The Operator's Manual only notes that the washer reservoir is "automatically heated".

As a note, engine coolant flows thru the reservoir coil all of the time in the 1995 E320.

Thanks.