I thought faulty wiring would be the culprit, but it turned out that it was simply forty-some years of accumulated dust, pine needles, and automotive goo.
there's a fan motor somewhere underneath all that dirt
I got the motor partially apart before realizing that complete disassembly would only be possible after removing the brittle plastic impeller. It's a friction fit, so I wet the shaft down with WD-40, twisted back and forth slowly and carefully, and used the mantra "please don't break, please don't break."
Once apart, I gave the motor and impeller a thorough cleaning, polished the commutator and contacts, and relubed the bushings.
back to life, running on a bench power supply
Back on the car, the ground wire was making an intermittent connection due to dirt and corrosion. The fan ground is connected to the nearby brake line union (where the brake light pressure switch is located).
bolt at upper left of brake union serves as chassis ground for fan
After a good cleaning all around, and some preventative Ilsco De-Ox on the contacts, the fan is back in business.
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