On a126 chassis there is a system to heat the lower portion of the windshield. A flat radiator panel is under the lower windshield where it receives hot coolant from a small dia. hose fitting on the bottom of the Monovalve. There is an inline thermostatic valve that regulates flow to the radiator panel, perhaps this same flow of hot coolant is used to heat the windshield washer fluid reservoir!
It would stand to reason that the heated lower windshield and heated coolant would be useful at the same time and under the same conditions, it would be very possible that both are connected and possibly share the same controller (the thermo valve)
On my 126 the valve is in the same compartment as the windshield wiper transmission and motor. The compartment is a space between the engine compartment interior firewall and the passenger compartment firewall accessed by removing the black plastic grill covering it.
So you can fix the thermo-valve if it is found to be malfunctioning.
You could always simply add an inline ball valve to the coolant circuit and manually control the heating of windshield wiper fluid.
Or you might check the monovalve itself, if you have not already done so. I saw a couple of other posts you have made concerning unwanted heat in the cabin, this is the default failure mode for monovalves, constant heat , much at idle, less at speed.
Continuously heated wiper fluid does not sound like a German design, so the control mechanisium is the logical fault. Good Luck!
There is always something to learn!
126 Windshield Defroster Heat Exchanger ?