My car is an OCC. I have an ongoing leak issue similar to yours. I have been chasing it for about 6 years. I have had the front glass resealed, because the glass urethane had detached from the body on the passenger's side, and there was some rust under the urethane. I re-resealed it myself with roofing cement along the bottom on the passenger's side after the reseal, and sealed every possible crack and crevice on the firewall while I had the front end off for installation of the 454. Each time I seal everything I can find, I hope the water stops, and am disappointed. I still get a steady stream of water into the passenger's side of the car. It settles in the rear footwell, like yours.
Recently, I removed my carpet, and have painted the inside of the firewall, and some other areas with watercolor paint.
Last spring, I tried putting a small shop vac in the car. I sealed the suction hose and wire at a rear wing window, sealed the HVAC intake, and turned the vac on. Turning the HVAC fan on can accomplish the same result. I put bubble liquid from the local dollar store everywhere I could think of that might be a leak, and nothing...no bubbles. It still leaks. I also tried using a smoke bomb, and turning the HVAC fan on, and got smoke coming out of everywhere, including the door seals. I could not see specifically where the smoke was coming from, because it came out in large volumes from almost everywhere. If you used suction, you could put water on the outside, and it would suck into any leaks. It would be like testing the intake manifold for leaks with the engine running. Using different food color shades for different areas would tell you where the leak is.
The only area I can think of, that I did not seal are the sides of the front glass. I also did not put sealant in the groove of the top door seals that go over the pinch weld.
Another possible place for water intrusion is the door hinge attachment point. The "bolts" that hold the hinge are attached to a plate, and the holes are larger than the bolts, to allow for alignment. If the hinge is not tight against the body, or the holes are exposed at the top of the hinge, water could enter there...I just thought about that, and have not sealed that point.
I just replaced the vista glass two years ago, so I am reluctant to think that it is leaking.
I also have a small leak at the driver's side rear door, and my seatbelt gets wet at the top anchor. (this happens on both of my OCCs, and may be related to the missing sealant on the top door seal over the pinch weld)
My car is parked slightly nose up at this time. I do not know if that has any affect on leaks.
It just rained tonight, and I will check it tomorrow for runs in the paint. I can only hope that paint runs will lead me to the leak.
Hopefully these ideas will help you find your leak.
1991 OCC 461 (.030 over 454) BBC, 3.23 posi, flash to pass, drop spindles & springs, Impala rims, Hydroboost, Recaros, MOMO/wood SW w/QR, custom wood shift knob, Pioneer DEH P77DH
1992 OCC now with 5.7 tbi, DEH P77DH
For a parts list, check https://www.impalassforum.com/vBulle...ion-parts.html