Sounds like your duovalve is letting hot water pass on the drivers side. How is the ac when you start the car cold first thing in the morning?

FYI Mercedes w210 does not use a blend door to direct air through/around the heater core like most other cars. All the air passes across the heater core. An electric valve regulates the flow of hot coolant into the heater core to selectively warm the air passing through the hvac system. The w210 uses a duovalve so it can seperately warm the drivers side and passenger side of the heater core. Try clamping off the heater hose feeding the duovalve. If the ac works ok, then diagnose the duovalve to determine whether you have a control problem or a failed duovalve. Given the age of the car, most likely it is the latter, but best to be sure before throwing parts at it. Good luck.