I know that on my W126 560SEL, there was a heat exchanger that was located under the windshield in the area where the wipers parked. This was fed by a coolant line that "y"ed from the return line from the heater core (after the monovalve). The presumed purpose was to keep the wipers from freezing to the glass.

Repair would no doubt require removal of the windshield.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles