The back corners of the wagon headliner are held by brackets. You can see one of the bracket mounting screws in the first picture from @HWKWallis. It is above the upper right corner of the opening. I think that is what they have you pound on in the manual.
In any case, the 50 year headliner is very fragile and the bracket would have to be bent out of position, which could be possible, to remove the fabric that is tucked into the corner. Doing so would poke holes in the headliner.
It is possible to use small bent hooks to reach through the perforations and pull the headliner down in the middle of the rear opening. At that point you can grab the plastic strip and open a gap to work through.
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