Im not sure about the 300D but my 560SL has a small electric circulation pump right before the monovalve. Its purpose is only to boost the heater circulation at idel and is not required during normal driving. It may be possible that yours is out and when you started driving the engines water pump took over. Also there is a temperature switch on my intake manifold which shuts off the blower motor in the heating mode but not the defrost mode. Im not sure if this switch also controls the mono valve. But you can check the switch operation by seeing if the blower motor turns off in the heating mode when the engine is cold and then it should turn on when the temp reaches I belive 40C. Also low water level will effect the operation of these switches. If the level is low the switch may be dry especially at idel.

John Roncallo