1997 LX 4-door sedan.

I have an increasingly annoying problem with leaks. Whenever it rains overnight, I find the next morning that I have standing water in the foot wells of the back seat. It started as a bit of damp about a year ago, and now I can float toy boats. Since it happens when the car is parked, I figured it leaks around the doors, but I cannot find any evidence of wetness on the seals. Any ideas?

Thanks,
Ben G