1998 Ford Windstar -Front A-C blows out defrost - Rear control blows heat - not cool on ac rear selector

These vehicles came with 2 types of ac controls.
The vacuum control system uses vacuum to actuate the vents and ducting from hot to cold. A lot of times the HVAC vacuum line in the engine bay (black hard plastic) is either broken or disconnected from its vacuum source.
The electronic vent control: does everything at the control knob via vent actuators and the control panel itself. With this one it could be any of the components. You would have to locate them and verify operation by moving temp/vent selector.

CHECK YOUR HEATER RELAY SWITCH AND YOUR FUSES

i found lose vacuum line and started geting cold air front was working until my car start staling and check engine light came on car turn off code po172 ....some were is

The problem is the rear A/C door is not switching from heat to cold.There is most likely a vacuum leak.I have the same van and the same problem.Another possiblilty is the switch on the dashboard.It can also leak.

the vacuum line in broken, check for same, start fromthe engine compartment, check to make sure the a/c doors are not jammed. check all fuse and relays

no, there is one service port for the ac system itself, make sure that the actuator doors are functioning properly and that all control knobs are set for cool air, your problem lies somewhere between the the two airducts that connect from the main source of cool air,

check to see if there is enough freon in the system,if so eplace the rear expansion valve

you need to recharge the rear AC unit

the blendor actuators strip out the gear inside the actuator cases on these all the time. replace the actuator.

If you have the heater directed to the windshield, the AC will turn on. Move the control away from defrost.