How do you replace a heater core in 1978 Ford pickup with factory AC?

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.