When you get the rail off you should be able to see where the water ingress has started but not always sorry. Make sure you squirt a bit of loose mastic in each screw hole.
You can always drill extra holes like In between each screw hole.
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I don't have much experience of window seals but if you use the search button on this website there's plenty info.
Only time I had a window leak in a fairly new van, I pulled back the rubber seal at the outside and ran a good squeeze of mastic around. That stopped the problem without the need to replace the seal.
Water is notoriously difficult to detect its source.
Where it gathers is not necessarily where it is coming in.
Well I resealed the rear awning rail today, used w4 tape and sikaflex where needed. And replaced the leaking window seal and rotten timber. Hardest part of the job!!!!, the plastic insert that goes in the window seal!!!!!. Not sure how to upload pictures. Doing the other side rear next Saturday