Darragh,
I ran into the same problem when I got to the Rear Retaining Rail. It appears that mine has some type of Yellow Adhesive holding the Retaining Rail to the Rear Bow. I pried gently with a Screwdriver to try to remove it, but to no avail. I also removed the Main/Large Door Speaker Covers to replace the speakers, and decided to bring them inside to clean them as well (they are filthy from dirty shoes hitting them over the years.
I may try a Heat Gun tomorrow on the rear retaining rail to see if I can get the adhesive to let go. Are all of these glued except the one in the video? Has anyone else had this same experience? My Headliner is FILTHY and Water Stained!