its been this way since i bought it. 1996 k2500 burb has ice cold air in rear vents(48 degrees) and cool air at dash vents( 65 degrees)all doors working ( checked them with a flexy camera).could seals on doors be bad or orfice in front evaporator be gone or clogged due to previous owners handywork? Any help is welcome before i rip it all out and start over. thanks