I pulled the carpet back, and put black watercolor paint in a line around the foot well. If it leaks, the paint will run, and then you can locate the direction of the source. Apply paint higher in that general area/direction to track it to its source.
I have had the urethane under the front glass deteriorate, but it never leaked to the floor.
My 96 SS had a leak in the driver side fire wall body seam where they applied body seam caulk. The dealer could not find it. The extended warranty would not cover it. I found it using a garden hose. Fixed it with some RTV.
I went crazy on my ‘94 trying to find a leak like that. I finally put a garden hose running across the windshield for a few minutes and it started leaking inside. I found out the water was traveling down the hood release cable and back in through the driver side firewall where the grommet had popped out. I rtv’ed the grommet back in place to fix it.
I had a real weird leak on my 95 RM sedan. BIG leak - passenger side aft - I pealed back the carpet after one rain and I am NOT kidding, there was over an inch of water on the floorpan; I sucked it out with a wet-dry vac. And I found out where it was getting in when I noticed a bit of ******-cruddy corrosion along the bottom of the quarter window above the weatherstrip; I scrubbed it down and laid a small bead of black Right Stuff along the top of the quarter window bottom weatherstripping - leak fixed.