2002 Ford Windstar only blowing hot

I guess the only question is. Do the rear HVAC controls work to control the blend door in the rear?

If they do it's probably a fuse or the Function Selector Switch Assembly. This isn't 100% though as IIRC the FSSA overrides rear HVAC controls until the REM receives a signal from the FSSA to allow rear HVAC control. There may also be multiple problems, such as a stuck blend door or failed actuator. You can rule out a vacuum problem as the blend door actuators are all electric and the vacuum controls only operate the recirc and airflow doors.

The two systems are independent with only some module signalling between them. The front and rear auxiliary HVAC are controlled by the FSSA, Instrument Cluster and the Rear Electronic Module, plus it's switches. When selecting a rear temperature or fan speed, the FSSA sends a voltage to the IC. Which then talks to the REM via a serial data line, to operate the blend door actuator.

The front blend door actuator is controlled by the FSSA and is a 3 wire setup with power, ground and control signal.

The rear blend door actuator is a little more complicated. It obtains power and ground from the REM. Has a position signal and two command signals for opening and closing the door.

The First thing I'd check is the fuse for the temperature select control. Fuse 10 in the Central Junction Box. If that fuse is OK you need to check the output at the temperature control connector on the back of the FSSA.

Honestly anything more other than doing some basic wiring checks or checking blend door actuators for power, ground and a control signal. You're probably going to need a scantool to communicate with both the IC and the REM. To verify module operation.