Ive had the same issue on my 110 CDI. first ive replaced the engine thermostat as engine temp never reached operational values (87'C). After that engine temp was OK gauge, was showing just above 80'C, still no heat.
Next Ive removed the pollen filter, which made huge difference in general airflow, but still no heat.
Next Ive removed circulation pump, tested with external power supply (12V), not working. Ive pulled it apart, found brushes worn down and not reaching rotor. Replaced bushes, works again, but still no heat.
Next Ive removed the 3 way water valve, there was no flow in any way! Pulled apart this one also, it has two brass plungers sealing against rubber surfaces, one for each way. cleaned up rubber faces, refitted. topped up coolant and now I have heating to Africa.
Quite honestly if I had to pay for all this, done by a shop, I would have been quite angry, thinking "No way that so much was wrong with a Mercedes" and thinking Ive been ripped off badly...
but guess what, its not over, found ventilation air recirculating flap not working either, I assume the actuator has broken membrane as it has vacuum leak.
Hopefully this helps others to get that heat back quicker!