My 2004 Expedition (Eddie Bauer) A/C has a poltergeist inside it. Whenever I put the A/C on Automatic and set the Temp the air flow immediately begins coming out the defrost vents then randomly shifts to the floor and dash vents before returning to the defrost vents where it stays. When I select the floor or dash vent buttons and manually set the fan speed, air flows from them briefly then goes to the defrost vents where it stays. I’ve replaced the A/C temp control unit TWICE and the problem persists. I need an exorcist to show me where I go from here to correct this problem.

 

Sounds like a vacuum leak in your vacuum system. The defrost position is the default position. Any vacuum leak where the damper control system can't hold vacuum, it will always end up in the defrost system. I would look under the dash for a loose or broken vacuum line. Especially the passenger side where folks tend to get their toes up under the dash.