trying to help the father in law out here he has a 92 F-250 and you can't switch the heater control from vent to defrost or floor, it has the 3 **** hvac controls and i pulled those out of the dash and really not much do it just a few vacuum lines and electrical plugs........where should i start?

 

You're doing this with the engine running, right? Fold the glove box fully down & find the 2 vacuum motors near the middle of the dash.

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If they aren't moving, follow their vacuum lines back to the control panel, and then out above the passenger footwell to the T in front of the accumulator under the hood until you find vacuum. A common leak is for the plastic reservoir glued to the evaporator cover under the silver heat shield (facing the R exhaust manifold) to break loose.

If it's a diesel, it'll have a vacuum pump on the engine going to a plastic manifold on one of the wheelwells. If it's gas, it ultimately goes to manifold vacuum.

If the vacuum motors DO move when you switch the controls, then the flaps inside may be broken loose.