A section of the stitching around my Boxster's window was broken when I bought it. At the time, my wife and I laboriously sewed it back in place with needle and thread. When an adjacent area of stitching failed a few years later, I used a sewing awl to stitch the repair. This method gave superior results and wasn't as difficult to use as well. I found that I it worked best to open the top to service position and disconnect the cables to the back of the top so that the inside as well and the outside of the window could be accessed from the back. Holding a thick piece of Styrofoam against the inside of the window to keep it from moving while inserting the needle from the outside made it possible to get more even stitches. This is by no means easy to do, but a section of broken stitching can indeed be repaired with the top in place.
Some more photos of my project here (near the bottom of the page) -
https://rx19jones.wixsite.com/projectboxster/projectboxster4
