The A/C and heat is regulated by a vacuum activated door under the dash called the actuator door. There may be a vacuum leak causing the door not to work. Check under the hood on the driver's side. There is a vacuum manifold that provides vacuum for several different functions inside and underneath the hood. One will be marked A/C and have an arrow leading to the proper hose. Check all the hoses for leaks because if one leaks it will affect all the vacuum for all the other functions. Vacuum leak may be the problem.
SOURCE: 1998 Mustang - AC blows only through Defrost vents
You probably have a vacuum leak on the passenger side. its a small plastic hose some times they come unpluged from the engine. or brake.
SOURCE: 1994 ford mustang 3.8 v6
Defrost is a default position for when there is a loss of vacuum. The vacuum controls the blend air doors. Start at the intake manifold and follow the vacuum hoses. There should be one that goes to a resivour (I think its behind a fender). It should T off and go inside the car. If the resivour is cracked or any of the lines are leaking that will cause the blower to only go through defrost.
I have also seen where ford uses plastic tubes and the dry battery acid had fallen on the tubes and ate holes in them. If any of the plastic tubes are broken, then each broken end can be inserted into a small enough rubber vacuum hose to repair them.
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