I just removed the heater core from an 82 300D. Needed to pull the dash. Heater core is found in the big black plastic box right in the center of the dash. It is attached to the ac condensor and the fan coming in from the left side in another plastic box.
A real pain to disentangle all of the wires and heater controls running around and through that big box. The core itself appears to be made from aluminum, or some sort of light metal. If you follow the heater hoses up to the center area of the firewall you can see just where the core is located.