I needed to soften the rubber(?) "flange" at the bottom of the rear window that retains the package tray in my '59 Chevy.
I don't think the car was ever inside a garage so the rubber was pretty stiff.
I gave it numerous coats of ArmorAll Ultra Shine.......the same stuff I treated the new vinyl roofs I had on a couple Model T's (one of which I still own).
The differences were amazing, especially on the new vinyl which became very soft, supple and fairly shiny after a single wipe down.
http://www.armorall.com/products/protectants/ultra-shine-protectant