Only get air flow from front vents no defrost

Again thanks so very much. I will try that after drilling a couple of holes in the heater box and see if I can blow any debris out of the vents.

Here we go again Steve and I thank you very much for all your help. I bought a snake camera and wondered how far from the duct work under the steering wheel is that door? More important now is this: I left Lowe's and got on Hwy 49 and truck would not go above 55 miles an hour. I realized I was driving in 2nd gear for a few miles. The low coolant light came on and when I got home the coolant reservoir was empty! I poured a gallon of coolant in it and it all leaked out. It is dripping from a bolt on the drivers side under the truck but heavily leaking above that bolt and I can't see from where. Any ideas of what has happened?