front ac vents not blowing cold air, rear ac vents blowing cold air.

I am experiencing the same thing on my Excursion. There is a blend door located behind the glove box to the upper left of the heater core. it has an electrical plug that should be visible. That can get crud in the servo part causing it to stick, thus not opening the blend door to the cold air. If this is not the problem, then move on to the accumulator. If only one line out of the top is cold when the compressor kicks in, then either the accumulator or the evaporator has the issue. By the way, the Haynes manual is not much help...