The vacuum manifold for the climate control actuator may develop a leak, This can cause poor or no operation of the air delivery and temperature control flaps.Google+
Car Problem Reports
Mercedes-Benz C280 Control Flaps May Fail Due to Leaking Vacuum Manifold for Climate Control Actuator
Mercedes-Benz C280 Problem
Average Mileage: 119,910 mi (60,000 mi - 175,000 mi)
I had this problem last year. It was solved by reconnecting a vac line that was near the oil filter. The line was not immediately apparent but it drops down in the oil filter area to a connector which can inadvertently pulled loose during a filter change. When I found the line, I pulled slightly on it and it was apparent it was loose. I think I had to reconnect it from under the car. When reconnected, everything worked normally again.
Visitor, 1999 Mercedes-Benz C280
passenger side vents suddenly start blowing cold air when heat is on. Driver side vents still blow hot air.
Visitor, 1995 Mercedes-Benz C280
A/C blows fine, but blower is stuck in defrost mode, and won't blow out the regular vents.
Visitor, 1996 Mercedes-Benz C280, 148,376 mi
had cracked vacuum block replaced but air only blows hot air
Visitor, 2007 Mercedes-Benz C280, 70,000 mi
Poor operation of the air circulation and temperature when turn to the lowest or highest degree
Visitor, 1995 Mercedes-Benz C280, 154,000 mi
STILL NOT WORKING
Air does not flow from the center vents. MB dealer said it was the air-conditioning actuator, which needs to be replaced. They said it is a $600 to $800 repair at the dealership and could be fixed for much less at an independent shop (recommended that due to the car's age and value, especially since its not a critical safety system).