[trevota]

are you sure the blend door is moving? Im not 100% sure how these heaters work, but if there is something that allows you to choose hot or cold air in the cab, i would check that.

Could also be an air lock or not enough coolant. been a while since i looked at my truck but i recall the heater hoses being pretty high up, close to the top of rad.

also if your truck over heats when it shouldnt ( like with a 195 t stat in 20 deg weather), then i would check for a possible water pump problem, or i have seen bad head gaskets/ cracked heads cause no cab heat and over heating. The cooling system pressurizes and that can restrict flow. That usually also has other sytpoms like leaks etc

My 65 with basic heater is rarely off of first or second fan speed cause the cab is so hot