Use citric acid (usually available at your local Merc dealer) to flush the cooling system. When complete disconnect the inlet and outlet side of the heater matrix and try to flush with a water hose, both ways. If still blocked then use compressed air to try and blow the blockage out.

Did you test the electric water pump? When you switch on the heater controls the duo valves should open and the electric water pump must run.

Also remember that the water flows from the back of the engine, through the heater matrix and then out at the duo valve to the electric water pump and finally into the radiator hose on the right side of the engine compartment.

Trust this might be of some use.