In most of these older vehicles the heater core is behind the dash on the passengers side. Check the floor board with the heater on to see if there is antifreeze or water dripping into the floor on the passenger side. If there is, that is where your heater core is located. It will not be fun to get to it. I am about 95% sure that this will be where yours is located.

This is the instructions it has them with air, so if you don't have air just ignor that part: Disconnect the battery ground cable. Remove the air cleaner. Drain the engine coolant below the level of the heater core. Remove the heater hoses from the heater core. Remove the screw fastening the A/C hose support bracket to the firewall. The expansion valve is located on the right side of the evaporator core housing. Remove the insulating tape covering the valve. Remove the two screws holding the plate over the opening that the A/C hoses exit the evaporator housing. Remove the plate and the seal. Remove the glove box liner. Separate the A/C duct from the register in the instrument panel and free the clip holding it to the plenum. Remove the duct from under the dash. Remove the vacuum hose from the vacuum motor on the right hand side of the air box. Remove the rear housing cover from the evaporator housing. The cover is retained by 4 nuts and one bolt. Disconnect the fresh air tube from the rear cover. After the cover is removed reinstall one nut to secure the evaporator housing to the firewall. Gently remove the capillary tube from the evaporator core. Disconnect the wires from the icing sensor. Remove the plate the sensor is mounted to. Remove the plenum by extracting the two screws above the transmission tunnel that hold the plenum to the dash. Remove the screws securing the evaporator. Pull the evaporator from the case and secure it above the case. Remove the heater core. Remove the heater air control door. There are no screws this is retained by a spring clip and snaps off. Remove the arm that supports the air control door and the retainer that secures the pivot arm. Remove the blower motor. Transfer the blower wheel to the new motor. Reverse the disassembly steps to assemble the heater A/C box.