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On 78 it's a cable. Mine was detached from the lever under the steering wheel it's supposed to move. I pulled the center console, cleaned and lubed and reattached the cable and it still doesn't work great, I have to finish moving the lever under the dash by hand.
The heater valve in the engine compartment is controlled by vacuum, but should fail to heat on. I'd start looking at the cable if I were you.
I was thinking of replacing the cable with a control rod from a model airplane since they serve the same purpose and are usually much smoother. If you can't find the PET diagram on line, let me know and I'll post a copy of that page.
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