The heater/outside air intake is in the 'cowl' panel, directly under the windshield.
Look for the cabin filter, and that's the entry point for air going to the heat/cooling.
The Owners Manual will have illustrated instructions on finding and replacing the cabin filter to help get you started.
See "Related Questions" below for more
air flow box
The fuse for the heater, AC blower on the Ford Explorer is under the fuse box next to the master cylinder reservoir. It is fuse number 36.
Freon is added in the low pressure port of the ac system. It is on an aluminum pipe on the passengers side, close to the ac drier.
Can't say for positive sure, but I would expect the Freestar/Monterey blend door actuator motor to be much like the Windstar. Common failure, easy replacement - about $40 See "Related Questions" below for links to the Windstar related blend door discussions
Under the cowl on the passenger side.
did the heater core start leaking? anyway you should have no problem with the air if you bypass the heater core.
Look for a little round blue plastic valve the size of a penny on the lines somewhere accessible. generally near compressor.
On the passenger side of the engine, not too far from the heater fan.
The heater core is behind the AC evaporator core. You must remove the panel with the venting on the passenger side. Then you must disconnect the two AC fittings inside the engine firewall. You do not have to remove the AC evaporator core, it will swing somewhat out of the way so that you can get to the heater core. There are several clips and screws holding the heater core in place that must be removed. Then you are able to remove the heater core. It is about a $35.00 item.
The heater core is located in the dash ,or should I say in the heater /ac plenum if you chase out the heater hoses from under the hood you'll get a general idea where it is located.
You can find a 1987 Ford E1 50 vacuum operated heater control diagram at your local Ford dealership. Most local libraries will have Ford service manuals with the diagram.
You probably have a leaking heater core.