Can be an easy fix or a full on pain in the arse.
Not easy to describe so recomend getting a service manual.
Start in the engine bay. Check the vacuum lines. One or Two, cant remember. There about three ml in diameter and enter the fire wall behind the last manifold inlet (slightly down and right)
In side car. Vacuum lines come in above pax foot well. Two black lines come in and connect to a white plug with about 10 plastic vacuum lines. multi color. one side goes to the climate control unit in the dash. The other side goes to the air outlets where the vacuum opperates the flaps (what ever there called). Check all lines are connected to plug.
Climate control unit. May be broken or leaking inside unit. Vacuum lines may be loose or off.
At each air outlet there will be a flap controlled by the vacuum to direct air. One each side (left and right side with two valves) of dash. Three or two centre behined dash. Check they work and lines are connected. i had two lines squashed by the aircon unit and something else which ensured I always had air on the windscreen. The vacuum reservior is located in to the right of the accelerator peddle between the inner and out skin. A vacuum line should come from the climate control unit to it and reture. Check this.
I had two remove the whole dash to check bits so it can take some time. Also did this some time ago and working from memory, so some info or all may be incorrect. This should give you an idea of whats involved. Either way it will take you some time to fix or alot of money to pay someone else to fix
As I said not easy to describe, especially after a long day. Best of luck!
Ford Doctor. Who requires a transfusion?