I don't know if this is common knowledge or not. I haven't been on here in a couple years but to cut a long story short, I had to junk my old 95XJ because the floor pan fell out. So i got hold of a 93XJ and I've just spent the last 2 months practically stripping it down to the shell and rustproofing.
I found that under the carpet of both my XJs was saturated so they obviously had a leak into both the driver and passenger footwells.
After a lot of time spent with a hose I finally found where the water was getting in:
On the driver side (passenger US) it was trickling in thorough the grommet that passes through the inner wing for the radio antenna harness (remove and smear with sealer to fix).
On the other side... (It was tricky!) I eventually found that an accumulation of dirt was causing water to leak in under the heater motor. I think on the US versions you have the brake servo there either way there's only about a half inch of clearance for the water to run down between it and the inner wheel arch so any dirt there causes the water to seep through the firewall under the motor/servo.
Ok that was a short story made long.. But the moral is make sure you keep the area where the firewall meets the inner wings clean to avoid leaks and rotten floor pans.
Hope this helps