Both 1970-1974 Capris and first-generation RX-7s have nice unitary heaters - a small box with the heater core, flapper valves, and various spigots for hoses to push onto. The setup isn't all that uncommon, but I'm familiar with those two.
The old Capris had an interesting heater setup. There was a fan switch and two sliders.
One slider was for temperature; it controlled temperature by either shutting the heater off completely or blending cooler outside air from the base of the windshield, so it moved a *lot* of air through the car instead of just recirculating it and restricting water flow to control temperature.
The other slider controlled the proportion of air going to the defroster vents and the dash/floor vents. You could put *all* the hot air to the windshield. I've never owned anything that could defrost a windshield like an old Capri.
Compared to the maze of cables, vacuum motors, hoses, and whatnots under a late-model dash, the Capri was a model of simplicity; two cables, two flappers, a box, and a heater core.
Unfortunately you're talking about something that's pushing 40 years old, long vanished from most salvage yards. The RX-7 heater is a bit smaller, but more "modern" and complex. First generation RXs are a lot more common, and there are lots of parts cars out there left over from Locost and Spec RX7 builds.
Once you yank the HVAC mess out of your donor and spread it out to look at, consider simplifying it to work something like a Capri. You're probably not going to want the air conditioner core and most of the weird stuff. The housing is usually plastic or fiberglass; there's nothing to stop you from sawing off unwanted bits and patching them over.
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