On the 1989 Aerostar it is under the dash near the passenger door.
the fuse panel should be either under dash on left side or if you open the hood it will be right by the hinges for the hood it will be a little black box or open your two front doors it may be where your doors meet the dash
A vacuum leak, or one of the doors is sticking/bad vacuum motor in the heater box.
Under the dash left of steering wheel
under dash between radio and steering wheel.
I have a 1995 Aerostar and the switch is on the passenger side, inside the door, down below the dash, behind the moulding. Hopes that this helps.
There is a cdoe programed in to the vans "brain" and it is telling you that there is something wrong. i have fixed this in a 1994 Aerostar. The dealer will know what to do from there.
Located under dash on drivers side near the door
Under tyhe dash driver side
On a 1997 Ford Aerostar : The fuse box is under the drivers side of the dash
Behind the dash is the HVAC housing. The blend doors are in the very center of the box. The dash and HVAC will need removed to access.Behind the dash is the HVAC housing. The blend doors are in the very center of the box. The dash and HVAC will need removed to access.
4 inch in the dash 6 inch in doors and back 4 inch in the dash 6 inch in doors and back