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In the older pickups, there are ducts behind the dash with baffles in them that direct the air to the dash, vent, and floor. These are operated by vacuum on a vacuum actuator. The baffles themselves were made out of plastic which over time get brittle and end up breaking the "hinge" that was just a pinched piece of plastic. When this happens, the baffle normally gets stuck and only blows out one place.
This happened on my 1986 F150. My fix was to disconnect the vacuum line going to the "vent" actuator that was attached to the broken baffle (located at the bottom of the dash in the middle of the trans hump). Next, I reached into the heater core access door behind the glove box toward the middle of the dash where I could manually push the baffle to block off the vent. Then, I could switch between floor and dash without it getting stuck on vent.
