Heater core replacement

If I remember correctly, the entire dash assembly has to come out to replace a heater core? I dont have A/C, so that helps.

Now that the temperature has gotten super cold (-10F), the heat in my truck is not all that good, I think I have a bunch of scale buildup in my heater core plugging part of it up, the truck has 300,000+KM on it, when it was only 0-10F, the heat was great. My heater valve is working, both heater core hoses are hot, thermostat is working properly. If I have to remove the entire dash, I wont bother, its not leaking, and its keeping my windows defrosted, plus I have a new car that has good heat for the cold days. It could also be that my truck has no insulation on the cab, I have no carpet or anything on the back wall, standard door panels, I do notice the defrost vents blow luke warm, where the floor vents are fairly warm, so I guess the cold is cooling down the defrost ducts and the warm air before it can leave the defrost vents.

This sounds like either the flap/door is not directing all the air through the heater core, or that some cold outside air is mixing with the heated air.

I'd have someone move all those dashboard controls while you stick your head underdash and look up to the sides of the control box. Do this on both driver and passenger sides. These are all cable-operated, and sometimes the little loop comes off the operating lever.