On a w124, there is no heat control/thermostat/valves/electrical circuit to control the heat of the windshield washer fluid. Engine coolant passes through the element without any control. Eventually, the windshield washer fluid is at the same temperature (or close to) as the engine coolant.
Although this seems to be a well thought system, it has it,s disadvantages.
Normally, windshield washer fluid is a mixture of alcool (wood) and water. Up north (Canada), windshild washer fluid is designed (mixed) to sustain a -40degF temperature, which implies a larger proportion (%) of alcool.
Since alcool vaporises at a lower temperature than water, in the middle of winter, most of the alcool is vaporised at normal engine operating temperature. With the heating system, the water do not freeze but as soon as you spray the liquid unto the winshield, it freezes instantly.since it is almost pure water.