its possible that the heater hot/cold blend door in the heater is broken (Common problem with rangers and explorers) I just replaced mine for the same problem. The door is plastic and the plastic fatigues at the socket where the control motor attaches to the door and breaks. The instructions say to remove the HVAC plenum (Heater assembly) but with a little patience and the proper parts and tools it can be done without removing the dash to get to it. To see if this is the problem remove the glove box to access the heater temp control servo motor (white box w/4wire connector) You have to pry it off the mounting pegs - gently! turn on the ignition and heater, move the temp knob and watch the peg in the bottom of the motor, it should turn slowly. If it does then the door is broken. If it doesn't then the servo motor has konked out. Before you put it together feel where the peg fits into the blend door. if you feel any rough edges in the D shaped socket the door is broken and you will have to replace the door.
If it is the blend door there is a simple fix for that. It's called the nail fix. Mine did the same thing but my door broke in the hot position. I removed the blend door actuator to make sure it was good. The top piece of the blend door had broken off. I measured the diameter of the hole that the actuator shaft sits in and cut a nail that length. Then I took a 1/16 drill bit and drilled a hole 3/8" from the bottom of the motor axle. I reinstalled the actuator and it works like brand new. I saw this on "the rangerstation.com"