Those are correct, I was a little confused at first too. The zipper is not attached to the backing, the backing creates the pocket that slides onto the headliner (like a pillow case, and is hidden when you re-install the headliner), the Velcro that affixes to the headliner closes off the fourth side. It is hard to slide into place because th hook Velcro catches on the headliner unless you have a long enough strip of loop Velcro to keep from getting hung up on the headliner. I wish I would have taken more pictures during the install, I didn’t take a picture of the roof side of the headliner.
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Much obliged! I had wondered why you removed the headliner completely. The "pillow case" description makes sense.