I had this problem with my 98 Windstar and all I had to do was replace the switch that is attached to the rightmost control knob (shown by picture from Wild Starchild). Basically what happens is that the switch no longer holds a vacuum and needs to be replaced. Just install a new switch and reconnect the vacuum lines and you will have cold air again. The switch is what opens up the lever to shut off the hot air and redirect cold air to the back. | 
