How to fix heater in Mercedes Sprinte 311 2001?

Ok anyone out there Van heater is blowing cold air no hot, the switch inside turns to hot but still blows cold, I have had front of dash off and cables are connected, (they were'nt though ?) I have had bonnet up and hot water pipe is hot and goes to heater I am assuming wire switch is not pulling something to open hot air vent maybe? I bought the van like this and I have no manual (this is not good) hope someone can give some frendly advice cheers