Aftermarket rear glass weatherstripping is ill fitting.
Do everything you can to save your original, or better yet leave it undisturbed.
I still have the original in my car, so they do certainly last.
Saw a 69 Shelby at Barrett Jackson a few years ago with repro rear glass weatherstripping.
The trim wouldn't fit over it right and was beat from mallet attempts.
What a shame for a car that sold for 99K IIRC, it was a rare kind of dove grey color too.