quarter window leak 1973 mach 1

I suspect that water is coming in from a gap between the roof rail in the quarter window area...it appears that there is a gap between the roof rail and the body of the car....

any suggestions on how to even begin a repair project like this?

I haven't seen not one youtube video on this particular type of repair.....plenty of window installations....but in those videos, not one person ever messes with the roof rail...

I ordered a new Mach 1 in 1973. The day I picked it up huge rain storm I had 2" of water in the back floor from roof rail leak. The farted around trying to adjust the rear window. I have power windows so the rears do roll down. Ford used a real sorry foam tape to half ass seal it out. Clean for assembly line but not very good. If you get NPD catalog they actually tell you not to use the foam tape but use 3-M strip calk which is what I have used for year. It never dries out flexed and there are so many ups and downs in that roof area it needs to conform easily. Here are some pictures of my car. It also has a vinyl roof which has to seal off to the roof rail also. What was really bad is my build sheet was under the carpet and pad on the side that got water so it was ruined the first day I owned. Those big rectangle holes for locating the roof rail are most likely where leak is but you can see all the layers of steel that has to seal. Be sure you get Daniel Carpenter seals they fit perfect. When you take the roof rail off put a screw driver in the screw and hit a couple licks with a hammer to break the rust and use new screws.