Neither of the rear quater windows would wind down. The right side just seemed stiff or jammed. The winder on left side appeared broken or disconnected from the window up/down mechanism. Inside panels of for an investigation!
Removed the winder mechanisms. Both seemed absolutely fine, not broken or worn out but perhaps suffering from 40+ year old gooey grease. A full clean, re-grease, pop them back, reconnect, spray the rubber reals with silicone lubricant and things are feeling better.
On the right side, there was an additional problem with a mis-aligned screw in the the inside seal that was jamming the window. Removed the screw and all is fine. The seal is on the inside, made of a felt like material so is not really sealing anything.