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Vent controls are operated by vacuum "motors"
Defrost is the "default" vent selection, i.e. no vacuum = Defrost only
The Vacuum it uses comes from the intake manifold, like power brake assist uses.
For vent control this vacuum is stored in the vacuum reservoir, a black plastic ball, usually located down low at the front of the engine bay passenger side, often best seen from under the truck.
From the vacuum reservoir there will be a black plastic vacuum line, looks like a wire, that runs into the firewall on passenger side, next to blower motor, you will see a grey plastic vacuum line with it, thats for Heater hose valve in engine bay.
The Black plastic vacuum line is often melted over time by exhaust heat so you loose vacuum in the cab, or it simply gets brittle and breaks
This line can be repair with regular rubber vacuum hose, the hard plastic hoses are just cheaper for Ford to use.
No clamps are needed since it is "vacuum" inside, so it "pulls" connection tighter it doesn't try to push it off, just get correct size rubber line to slide over the hard plastic line, cut a section out to take with you to auto parts store.
So I would start by first finding that black plastic line from firewall to reservoir and make sure it is OK
The Vacuum reservoirs have been know to crack and leak just from age, and even minor front end collisions can damage these.
The reservoir gets vacuum from the intake manifold and this line could have been unplugged after engine work, so once you have found the reservoir trace its other line back up to intake manifold