[Roadsters] 69 2000, no rear blinkers, but fronts work, ideas?

[O'Farrell, Fergus P [US] (AS)]

69 2000, all stock wiring. (OK, 55 amp GM alternator connected into stock wiring harness with 30A fusible link)

All tailites work, (running and brake) and front blinkers working, but none of the rears working.
So power going to blinker circuit from switch, and blinker cans working, and all fuses are sound.
WTH?

Thanks,
Fergus O
(noticed today that it has been 3k miles since oil change a year ago, that shames me, needs to be at least 50% higher than that)
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