how do you get the little white window regulator button on the lift pin on the rotating cable? it looks like the side of the button in the third picture (narrow slot) goes over the two prongs shown in the second picture but i can't imagine how to make that happen.

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Carefully bend them

I thought about that, but then you can't bend them back after the button is on there. The tabs are there to hold the button in place.
Thinking about it this morning, maybe everything fits tight enough that it doesn't matter. Bending the tabs looks like the only option.

I was looking for my pics, but I was pretty sure the original tabs weren't bent out. At least on my car. I'll check later today.

My car is a December 1969 build and the tabs were bent out slightly to keep the plastic button on.

They look overly bent. Take a look at your other window winder.

Straightening the original tabs turned out to be the solution for me. The didn't both have to be totally straight, I left one bent more than the other but a bit of force popped it on.