No hot heat from vents

This morning was real cold 25F and i went to show the car to someone interested in buying it and i found that the heat and defrost both cold air. I replaced the cam phasers on this car few months ago and all was good, didn't drive it in the winter really, it was parked and drives excellent, i noticed the top radiator hose has no real pressure like it should, feels empty, engine coolant temperature is where it should be, no leaks and doesn't overheat, when i first cranked it in the am it said check coolant, they usually do that till its warm then message went away, checked reservoir after that and it was full, and message went away, still no heat, blower works on all speeds. I noticed when i turn the car off the piece in front of the reservoir is on for a while then shuts off, not sure what it's for but its an electric piece. Any help is appreciated, no codes under the ac system except what is in the picture. I have an 2008 Mercedes Benz c350 as well if parts are needed for trial.