[Bigg G]

I had the exact same problem on my 03 Chevy Astro. The door in your heater box is not opening to let the warm air in to your vents. After letting the truck run a bit turn off your engine but leave the switch in the on position. Slide or turn your heat / AC control lever from from the vent to the defrost position a few times while listening for movement in your heater box just below your dash in the center. You should hear it opening and closing. If you don't there's a pretty good chance you have a bent control cable (if you recently removed or installed your radio) or a cracked vac line.