I've located and repaired two leaks on my '93 Delta 88, so this may help:

1. Remove the rear seat cushion and check the floor pan underneath of the
padding. . I found an area about 1" x 3" that was paper thin and had many
pin holes in the metal. I ground it out and patched it with Bondo applied
over a piece of screen. Still holding up.

2. Assuming your car is a 4-door - there is a plastic piece at the top of
the B-pillar that sits just below the roof gutter and between the door
seals. I discovered that water was somehow getting under this molding-like
piece and then dripping down inside the B-pillar. The water eventually would
find its way back along the side of the body pan, and then collect in the
rear passenger footwell area. The result was a saturated pad under the
carpet, which took a couple of weeks to dry, even with the carpet pulled
back and a portable fan blowing on it.

Hope that helps...

doug