This is a future reference/tutorial post:
This problem is usually due to a bad blend door actuator. The Max AC setting bypasses the blend door, and when the actuator is broken, air pressure from the fan can move the door to the max heat position. Sometimes the actuator will just stick in one position, and so adjusting the temp dial will do nothing.
Here is a video outlining the location, removal and installation of the actuator. This guy makes it look easy; getting the actuator in and out is pretty difficult and time-consuming. Also, the actuator post may not be in the right position for getting it installed into the blend door socket.
If this happens, then you will want to reattach the environmental control panel (I removed it from the dash and plugged it in; did not have to hook up the vacuum lines), plug the actuator in, and turn on the vehicle (accessory will work, you don't have to start the engine). Adjusting the temperature control will cause the actuator post to turn to a different position.
I had to do this, and then ended up with the actuator partially inserted, but the blend door was pegged to the cold position and I couldn’t get the actuator placed, so I had to reconnect the cable and reposition the post again so that I could finish installing. It took me forty minutes with an assistant and a pair of chopsticks to get this connector reinstalled, so if you have to move the actuator post, don't disconnect it until you are done, if you can help it.