I have a W110, we have very similar HVAC systems.

Most likely your heater valves are not opening. Possibly the cables are broken or are slipping on the valves. Have you verified the valves actually move? They do have a small bypass built into them that allows a small amount of water through the heater core at all times. Have they ever been serviced? Its possible to reinstall the heater valves out of "phase" the valve may still move but the holes dont line up for water to flow. Be careful flushing heater cores with a hose. City water pressure can be very high, locally i have over 80psi. The heater core never sees that and can blow the joints on it.

There is a great write up on the Ponton website about heater valve servicing. Its kinda funny that you can take a spark plug and it will screw into the valve to pull it out.

Mercedes-Benz Ponton Heater Core Valve Repair © www.mbzponton.org

__________________
"It's what non-car people don't get. They see all cars as just ton-and-a-half, two tons of wires, glass, metal and rubber. That's all they see. People like you or I know, we have an unshakable belief that cars are living entities..... You can develop a relationship with a car, and that’s just what non-car people don't get."
-Jeremy Clarkson