Help with Air conditioning.

I have a 1988 GMC S15 truck with a 2.8 v6 automatic. The A/C blows cold, but it won't come out of just the vents. It comes out strong on the floor vent in the middle, but just barely blows out of the vents. It stays in this same mode even when I adjust it. Where should I start looking to fix this problem? Fuses, hoses, connections,etc....

Anyone? Anyone?

check your mode valves behind on the box itself(heater box) chances are thsi si stuck , i think there are 2 of them, or your valve doors are stuck in one posisition.