Years ago, I put a new headliner in my 73 Mach 1. I did it the "right way" originally, but the spray adhesive I used to attach the headliner material to the side panels came loose. I carefully cut the material away from the rear window seal, and reattached the material to the panels. When I reinstalled the panels, they did not tuck into place as they had before. This weekend I finally solved the problem by using some 1/16" thick black plastic strips 3/4" wide x 21" long. I tucked the strip into the rubber seal, leaving just a bit to hang over the panels and hold them in place. It looks like it was meant to be that way and is sure a lot easier than pulling out the rear window again!
Gary