The air blows hot n cold but only through defroster and in back in 99 expedition after I had a new compressor and alternator installed. Is it a fuse that controls the vents or a vacuum line? Where?
Identify your system, if it has cables vacuum automatic or a combination. Cars that divert to defrost often have lost vacuum. passenger side under the hood base of the windshield is the area to start removing parts for access. but if it has an electronic automatic system, EATC it can be run through self test and there are several inspections. you need a service manual or take it for a look see at the ford service drive for prompt and accurate answers.
Behind the glove compartment sometimes provides enough sight to find where mode doors are supplied by vacuum. electronic ones have actuators and sensors requiring inspect and adjust as testing progresses. Some stick and need replaced, ones told to go the wrong way could be compromised by sensor damaged or failed.
Look for a disconnected or broken vacuum line. The vacuum line you're looking for comes from the intake manifold on the engine and goes through the firewall into the passenger's compartment. When the system loses vacuum it defaults to defrost for safety reasons. HTH. - Jim
I was going to answer buut you have the right answers above. Follow their advice
Looking for a Used Expedition in your area?
CarGurus has 23,971 nationwide Expedition listings starting at $800.
Search Ford Expedition Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford Expedition Experts