Here is a photo of the plenum before the ceramic heater was put in. Taking it out meant removing the steering column and the entire instrument panel. Not fun, but certainly a learning experience.
Here is the original insides. There was evidence of a mouse having lived in there! It apparently come in through either the external or internal intake (recirculation intake) and made a comfortable nest next to the A/C evaporator. I don't know how long it had been abandoned. A spray down with bleach-water was in order, followed by a thorough vacuuming.
Following several examples of what others have done, I purchased an in expensive ceramic heater from local hardware store for $30. And promptly disassembled it.
The actual ceramic heater element is quite small compared to the heater core from the PT Cruiser. It is mounted on a plastic fan housing. I decided to keep the housing, sans fan, and mount it in the location previously occupied by the evaporator.
Once it was all back together, the baffle that shunts the air to the non-existent heater core was permanently closed. I plan to repurpose the dash knob that had been attached to it as a thermostat instead,
The wires go out through the evaporator drain. I'll orient the drain tube upwards when it is finally installed. The openings for the evaporator and heater core were plugged.