This condition was more exagerated on my car. In fact, one side would have the heat on full eventually (the driver's side, in a US car) while the other side would be on full cold. I would "fix" the problem by turning the defrost on for a few minutes and then off, and I would get ice cold air from both sides. For a while.

Turned out to be the "duovalve" for the hot water flow to the heater core on the driver's side. New duovalve and the problem went away. One of the guys on the Diesel Forum actually posted instructions on removing and cleaning the valves to fix the problem.

Hope this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)