Had same problem .I checked a few simple things like the coolant and hoses. When my mother in law said it never got warm in the truck either it made me think something was up with this truck. It wasent til I was laid out on my floor and had the glove box down Or open all the way and then some . I see the heater controll solid wire on top of the air box . So I see that it moves to the left to close off the cold air and to the right to open the air. Then I took my fingers and try to see if I could close it farther and their was my cool air or rather my warm air problem solved I adjusted the wire and now I can cook my kids feet with the vent . One screw and a few moments .After all these years.
1984 GMC 1/2 ton long box 6.2L has hot heat in 2005. And my temp gauge always said 190-200 still blew out luke warm air.