There is one other thing I neglected to mention, and that would be the little vacuum supply line. This tube is right near the refrig.lines and continues to your dash (through the fire wall) if theres a leak "hole" in the tube you will only get a/c through the defrost vents, these tubes also have been known to fall off completely which can also cause your problem. I'd suggest checking it out and making sure it is well connected. Sorry I overlooked this before but I remembered this happening to a mazda p/u that had the A/c through the def.vent problem that you previously described. | 
