96 Windstar, Heater Fan only works on High.

Could be your blower resistor. It limits the voltage to your motor to change the speed it blows at. Blowing at one speed only could mean a shorted resistor. Of course some vehicles dont have them also. Cant say for certain on yours

The resistor pack has resistors that set the fan voltage for low med and so on. The high setting uses no resistor so when the resistors burn out (common) it only runs on high.
The rear ac uses a vacuum solenoid to switch from heat to ac. On the back of your rear heat ac fan switch there's two vacuum lines, when in ac mode, the vacuum connects and draws a vacuum on the rear solenoid, mine did the same thing, I replaced the heat, ac switch and it worked fine.