I have a 98 Chevrolet Blazer with temperature control. Up until a few weeks ago it was working fine. Now, only the heat works and it's coming out all of the time. The amount of heat that comes out and where it comes out can still be adjusted. However, there is always some heat coming out even when it is off.
Temperature Control on 1998 Chevrolet Blazer
The temperature door actuator may have failed. If you can remove the actuator try moving the temp door by hand to see if the output temp will change, if it does the actuator may be at fault.