From 1985 MOTOR A/C & Heater Manual
Remove instrument panel. Remove defroster nozzle by removing 4 screws at cowl and screw at heater case.
Disconnect vacuum lines to programmer and actuators.
Disconnect harness leads and air mix door linkage at programmer.
From engine compartment, remove 4 heater case to cowl retaining nuts.
From under the instrument panel, remove heater case to cowl retaining screw. Remove heater case.
Remove 4 case to core retaining screws and heater core.
Reverse procedure to install.
Hope this helps