1996 Dodge Ram heater problem

It's possible that the temperature blend door is stuck. The shaft it swings on is metal, and over time they tend to rust and binde up, operated the temp knob while you look for or feel for the actuator that moves that door, some actuators ar vacuum operated, some electrical. Either these motors, called servo motors can go bad or the above mentioned issue! If the shaft is frozen, try a small drill bit at the base of the shaft, drill thru the plastic and spray some white lithium grease in there, if you can move it by hand, so much the better! Work it back and forth till it moves freely!