Are you just curious or do you have a heater problem? In US MBs there's usually an electric auxiliary pump between the heater control valve and the water pump inlet. The '95 E300 Diesel is the only US model I know that has a bypass line from the cylinder head cabin heat source to the monovalve. For the rest coolant doesn't flow from the cylinder head to the heater core when the heater control valve is closed. The auxiliary pump only runs when the heater control valve allows flow.

Sixto
83 300SD
98 E320 wagon