Oh, and the film on the windshield... Probably means that your heater core is slightly porous if not actually leaking. It can be porous enough to create that film without causing significant coolant loss, in other words much less loss than you are experiencing. But if you had no film at all I'd have placed the heater core lower on the list of suspects.

Pressure testing will tell the story.

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1987 W201 190D