I have a 2003 Siera pick up truck ans the heater is blowing out cool air and it only comes out the top not the vents what is my problem, I flush the radiator and put fresh anti-freeze and water in it

I tried flushing the radiator out and puting fresh anti-freeze and water in it

I put a heater control swicth in it and replaced the recister in it

Either needs a thermostat, or the cooling system needs to be purged from air pockets, and or the blend door actuator is not working to allow the blend door to close to the outside air.