...not sure about the S-class, but my 201 has a heater coolant valve under the cowl in about the same location that you are descibing. I think it's normally closed and has a vacuum diaphragm to open it when heat is called for by the Tempmatic system. There was a slight coolant leak coming down on the transmission from the heater box drain. After removing the plastic cowl screen to access the area I could clearly see that the valve was seeping, and it was easy to swap in a new one.

Duke