I have a problem with my 08 mustang v6. At first when I turned on the ac/heater on there would be a crackling noise. This noise would come up every time I switch from ac to the heater or to the defrost. One day this noise stopped. Now it's stuck on AC and doesn't switch to the heater or defrost. Anybody had a similar problem to this?

 

https://mustangforums.com/forum/2005...ctuator-2.html

It's going to be you temperature blend actuator most likely. If I recall correctly, it's the one on the center stack, passenger side, bottom. The top one controls dash/defrost vents see link for details