how do you stop the rear window from leaking in a convertible (mustang)

15 year old top. Probably time for a new one. Band aids can only do so much when the top is due for replacement.

Yes, but I will try the 3m glue first that you gave to another person

There is a seal underneath where the back window bolts on to the car that dry rots and leaks-I have a 2003 gt that was leaking there, $1650 dollars later (after a new top and weatherstrips) it still had the same leak.It is called the retainer plate seal, couldn’t get from the dealer but found it at parts depot for $36.Hope this helps someone else avoid the expense and frustration I had.Mine leaked out of both corners and would fill up the area behind the back seat.You can see the areas on the old gasket that were bad, it absorbed water like a sponge.