I have a 99 Grand Caravan, and the temp control just went hay wire. I'll have the temp selector on cold air and out of the blue you'll hear a brief motor running sound and hot air starts to blow. You then have to move the selector to the hot air setting and back to cold air(you can hear a motor running again)to get the cold air blowing again. Could it be a relay or maybe even a bad heater control board?
Heat Control on 1999 Dodge Grand Caravan
This might help out. Try recalibrating the system. First make sure the evaporator is warm. Start engine, set blower motor to highest speed. set mode so air comes out of dash-board vents only. set temp. controls to cold(all the way to left). Now depress and HOLD for 5 seconds, rear washer and rear wiper buttons at the same time or untill all LED's Lit. Now it is calibrating for minute or so. At the end of this process only one light should be blinking that will be rear wiper. Press this one to get out of programing mode.