I have an intermittent problem with my climate controller on the MB380SL. Sometimes it works fine and other time I have no heat or fan blower movement. One cause I’ve read on the online forums was a bad mono-valve that may intermittely become stuck and prevent heated coolant to pass through and into the heater core.

This valve receives its signal from the climate controller inside the cabin. You can see where the screen preventing contaminants from entering has become corroded over time.