this happens with me too! but it's the back footwell, behind the drivers side.
i have a 306...
i've looked around for leaks and things, but there's nothing that would explain it :S
does anyone else have the same problem?
I've posted about this under "damp seats", but here it is again.
Yes it is quite a serious water ingress problem.
Can I ask you first to lift up the rear seat (at the front first - then pivot) after rain - and see if there is a puddle or rust starting there.
If there is, the problem is the design of the rear light clusters. Take them out, and use silicone on the top areas when refitting. Wipe off excess with a non-fluffy rag when finished.
The other likely culprit is the windscreen seal. The only real way to check this is by playing water from a hose onto the screen, then looking for water ingress from the insiide. Silicone for that too, but wait for a dry spell before attempting it.
You can also check inside the bonnet on the LHS where the air intake is. Check all sealed welds are still sound and not cracked sealant.
If you still can't find out where, then you'll have to take your carpets out to find the source
Best of luck.