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Yes, you need them. B-bodied use one situated in the center of the rear roof frame and e-bodies use two. You will need them both to install your headliner correctly. They are made from a light gauge spring steel... the same material you find under a seat that ties the mechanisms for the sliders together. If you have something like that lying around you could just make them. The length should be such that they are secure between the frame and bow when the bow is pointing straight up (12 O'clock)

My '71 RR has two, not just one. They are positioned one left and one right from the center of the back glass.