I'm in the process of doing my 1st seat project. My uppers have a zipper back. Is this zipper functional? Like do I have to undo it to get the cover stretched over the new foam? Lookin at the original as I take it apart there no pull tab on the zipper so there is no way to unzip it. So either the cover was installed with zipper closed or the pull tab was pulled off after everything was complete....seems unlikely. Any help appreciated.