leaky convertible mustang gt

I have a 99 mustang gt that I bought new. It just recently started leaking from the rear passenger side around the rear window where it meets the quarter panel.

has anyone had a mustang that leaked?

any ideas on how to repair?

all suggestions are greatly appreciated.

I have an eclipse that used to leak when it rained heavy. I just went to cabela's and got the spray sealer that you use on canvas tents for camping and sprayed a whole can on the roof. No more leaking at. The water just beads up on it and runs off. worked out pretty slick for the whole six bux I spent on the spray so I would try that.

turns out the rear window curtain had signs of dry rot and water was leaking via the seems