My wife's Dodge Stratus developed a second heat control problem. The heater fan only worked on high. This has happened to me before on other vehicles but I could not locate the speed control assembly. Someone told me to check under the passengers side dash near the blower motor. It is almost impossible to see it there, without a mirror, but I was able to get a picture of it. First remove the flimsy nut that holds on the cover, then remove the cover, and then it is visible. Remove the two mounting screws and turn it around so you can see the back side to disconnect the electrical wires. It requires releasing two catches to be disconnected.
The fan speed control is the light gray thing on the lower right side of this picture.