We purchased the 2009 sport trac last night from family friends and noticed on our way home with it, when I switched the vent control from the front facing vents to defrost(it was humid and windshield fogging up) the air didn't switch from the front facing vents to defrost vents on top of dash. So I switched it to just the floor vents and same thing. AC(or air) only comes out of the regular front vents only. AC and heat both do work though. Is this something my son(he's almost 17) can diy or do we need to take it in? What is the possible cause? Thanks!