I have exactly the same problem with my 1997 E320. Last summer I took car to dealer and they said it needed a new evaporator - $3995, please. I decided to get a second opinion but never did. We are just now having warm enough weather to require A/C and the symptom is still there. If I had a leak in the evaporator, wouldn't the coolant have leaked out by now, leaving me in default EC mode? Doesn't the system run all the time?

I would say I get adequate cool air out of the right side of the center vent but that it is negated by the warm air coming out of the left half. Turning the left-hand thermostat down, helps but doesn't completely cure the problem. I(s the evaporator in two sections?