I have a 94 Chev S10 pickup with a A/C vent problem. The A/C works as in it blows cold air but it blows out the floor vents and the defrost vents, not the A/C vents. When I select the MAX A/C it blows out of the proper vents. If I select normal A/C it blows out the defroster and floor vents. If I select MAX A/C first then let it run a little while then change it over to normal A/C it blows through the proper vents. If I select just the vent it blows out through the defroster and floor vents. Is this a relay problem or a switch / vacuum line problem?

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1994 Chevrolet S10