Mercedes: no heat in mercedes ml320, mercedes ml320, top screws

Question
heater only blows cold air

Answer
Steve,
I had a similar issue with my 86 420SEL. The Monovalve was not working. Mercedes wanted 300 for a replacement after market was 190.00.
I did a search no EBay and found an OEM valve and bought it for $10.00.
The location on my vehicle was in the firewall bay. I decided that to try and remove all the hoses would cause more problem, so I just remove the top screws and installed the new mechanism in the original housing.
Problem solved.
Models vary, so I do not know how your is designed. But you may a failed valve, or an electrical lost to the valve.
I would check for a blown fuse first.
Good Luck
Rob