Water in trunk after heavy rain

I have a 76 Duster that had the same problem.It appeared the water was coming in at the front (below window) and leaking toward the rear,but was in fact coming in the rear and going forward.
as it turns out there was a slightly loose end on the trunk seal where the two ends meet and a small amount of rust was forming and lifting the gasket. I cleaned it up and reglued it and all is good.Good luck-rag