Remove rear side panel, safety belt mounting and rear screw holding the internal trim pad. The pad pulls slightly up and off, but there are 2 metal tabs hooked through holes in the side of the car which may have been bent over to hold it in place. reach up inside the side panel aperture and if these are bent up, straighten them with your fingers to release the pad from the car. Be careful with it, the moisture from the side window often rots the metal backing and it may break if it's too far gone. I'd never seen any sign of leaking from my car but there was still a bit of rust along the bottom of this metal that needed treating.
With the pad removed, you'll see the rubber seal is glued down to the car. Undo the little chrome plate from the bottom of door pillar chrome strip (Bpost). This holds the end of the seal down on the inside. Peel the inside of the seal off the glue.
Pull the rubber door window seal out of the B post channel so you can undo the 3 screws holding the quarter window hinge blocks. Undo the 3 screws that hold the popout catch to the C post. Take care the window doesn't fall out! Slide the window out of the B post channel and put it somewhere safe.
Peel the old rubber seal upwards and ease it out of the alloy trim along the side of the car. It is just hooked into this trim strip and comes out easily without disturbing the strip. Pull the narrower upper half of the seal out of the alloy channel in the roof. Clean the channels and trial fit the new seal in place. Pops' seals are good and long so a couple of inches will need to be removed to match the old one. The end of the seal needs to be trimmed to fit slightly inside the shape of the B post, and notched so it's inner and outer edges extend either side of the B post. But make sure it fits down flat where it sits inside the B Post or the window will be pushed up too far when it's refitted.
Glue the bottom section of the seal (and the car) with contact glue and when it's ready, tuck the seal down into the alloy trim on the outside and press down onto the glue on the window frame. If the alloy trim is too tight to get the rubber under, lever the strip very gently upwards with a ruler to increase the gap for the seal. (When it's all done you can gently push the alloy trim back down with a flat wooden block to trap the rubber tightly). Glue the top narrower length of the seal into the roof section and tuck the edges of the seal in with an old screwdriver or blunt knife, working slowly fron the rear corner towards the B post. Cut the top end of the seal into the shape of the B post.
Refit the interior pad to hold the seal down while the glue sets properly. The metal hooks in the back of the pad can be bent to pull it in tightly and the safety belt mounting and rear screw hold it front and back. Refit the little chrome square to hold down the end of the seal in the inside of the B post and refit the interior panel.
Refit the quarter window into the channel and line up the hingle blocks with the screw holes in the B Post. When the screws are in and the popout catch refitted, check the window for opening and closing and then refit the B post seal, tucking the edges in with an old blunt knife or screwdriver. (I used an old blunt stainless steel knife that has had the tip of the blade heated up and bent at rightangles).