The "monovalve", which controls coolant flow to the heater core is a common leakage item. Unless the HVAC system determines that heating is required it is off.

If it's leaking, coolant will drain into the HVAC plenum and through a drain above the transmission. If it's leaking you can can usually see coolant residue on the trans. If so, the monovalve is suspect and should be inspected for leakage, which will likely be readily apparent when you see it (wet). The heater core is the other suspect, but if it's leaking it will usually leave a film on the windshield and an antifreeze odor that you can smell when in heat or defog mode.

Not sure about your model, but on my 190 I have to remove the cowl HVAC inlet screen to see the monovalve - just a couple of screws or clips as I recall. The W/S wiper arm does not have to come off - just slip the screen out from under the rubber boot. The monovalve easy to R & I and a good opportunity to do a coolant change with Zerex G-05 if you are close to due.

Duke