Fan on air condtioner will not work ,vacuum from defrost error

I think this link will help
https://www.stangnet.com/mustang-forums/threads/a-c-problem-on-95-gt.641508/

Sounds like the diverter door is not working properly. Not sure where it's located or if it's vacuum or electrically actuated. It should be in the plenum under the dash but may not be accessible w/o removing part of the dash.

These selections are run by Engine vacuum. sounds lie a diaphragm is NG to control the lower door

It sounds like vacuum is not getting to the vent, heat and A/C controls. Find the vacuum can. It may be under the hood on one of the inner fenders. The "one-way" check valve is usually located in the line and close to the can, make sure it works properly. The valve could be the very thing that's causing the problem. Trace the lines back to the vacuum block at the intake manifold or fire-wall, and also back to where the line goes thru the fire-wall. Check the can for leaks or obvious holes. They used to be just a plain metal can, but it could be a plastic ball. Check the lines as you go, for breaks and rub-thru's. Move the controls thru all settings and listen for vent door movement. Worst case is a bad vacuum motor or air flow position selector switch, and/or a faulty Blend Door Actuator. If you have been driving around with the Temperature knob or control set at the same spot for months, move it from fully cold to fully hot several times and wait for the air temperature to change each time. This will help reset the blend door and vacuum control system. Hope this helps... Let me know what you find.

vacuum supply problem to the HVAC controls. check the vacuum hose leading from the engine to the passenger compartment.

check or loss of vacuum to mode control switch in dash ac control. these are not blend doors, they are air flow control, a blend door controls the air temperature not where it comes out.

the ac blend door is out of adjustment or the the actuator is not working look behind the glove box and watch the arm move

check the vacuum lines in the selector switch. the baffles that switch from hot air and cool air are vacuum controlled and if the lines are detached or cracked the suction will not move the baffles.

hi, seem you have vacuum problem,open your hood look for your vacuum distributor maybe have disconnected vacuum hose,ty

First thing I would do is rig a vacuum cleaner hose that will go deep into the defrost vents... cut the defrost on HIGH - WIDE OPEN - FAN ON HIGH!! -- then stick the hose down and dig around.. you will find you have collected a LOT of farm "dust" (down)(the way dust falls) in the defrost vents.. and the A/C vents are not (down) and nothing falls (down into) the vents...