Could be a vacuum leak somewhere behind the dash, and the blend door is not closing all the way in one direction or the other. Just have to poke around there behind the glove box with the car running and listen for hissing. Unplug vacuum lines back there one at a time, see if anything changes; see if the temp changes with the line only pulled, and then plug the line with your finger and see if that affects it. You might have to go as far as pulling out the HVAC control deck to see if something has come loose or split or something from one of the vacuum lines back there.
I'm not sure if Foxes have the same setup as Panthers, but on Crown Vics they tend to default to a full-heat setting when there's a vacuum leak. Be thankful that Fox Mustangs don't have that stupid, STUPID Electronic Automatic Temperature Control (EATC) system, like what I've got on my Town Car. Both of my hands look like a rabid tomcat got hold of me from all the scratches and missing chunks of skin I obtained from trying to remove that STUPID heater blend door actuator - basically, some dimwit at Ford thought it would be "cool" to use an electronic servo (motor) to open/close the heater door, instead of a reliable and simple vacuum actuator. Instead of replacing the actuator, I just manually moved the blend door lever to full heat and wedged it there; come Springtime, I'll just reach in behind the glove box, move it back to the cool position, and wedge it into place for another nine months.
If it turns out to be a vacuum line issue on yours, just get some hard plastic line from Autozone or wherever for a few bucks and splice it in; if it's the vacuum actuator, maybe check out 50resto.com or the boneyards and pick up a good one, probably less than $30.
I'm not sure if Foxes have the same setup as Panthers, but on Crown Vics they tend to default to a full-heat setting when there's a vacuum leak. Be thankful that Fox Mustangs don't have that stupid, STUPID Electronic Automatic Temperature Control (EATC) system, like what I've got on my Town Car. Both of my hands look like a rabid tomcat got hold of me from all the scratches and missing chunks of skin I obtained from trying to remove that STUPID heater blend door actuator - basically, some dimwit at Ford thought it would be "cool" to use an electronic servo (motor) to open/close the heater door, instead of a reliable and simple vacuum actuator. Instead of replacing the actuator, I just manually moved the blend door lever to full heat and wedged it there; come Springtime, I'll just reach in behind the glove box, move it back to the cool position, and wedge it into place for another nine months.
If it turns out to be a vacuum line issue on yours, just get some hard plastic line from Autozone or wherever for a few bucks and splice it in; if it's the vacuum actuator, maybe check out 50resto.com or the boneyards and pick up a good one, probably less than $30.