Ciao

(Maybe this is already solved aber doppel haelt besser).
I have been reading some posts from guys speaking of really wet floors
(when it rains). I owe a 306 1.9 D and had the same problem but a
smart (italian   ) guy solved it!

No window seal broken, no air conditionin leak, no condense water.
The front steel (?) panel of the 306 (open the  bonnet and you will
see it at the height of the passenger seat) has a hole to let pipes
and conducts go through (I assume its basically aircon pipes). That
hole is selaed with a gum sealing that eventually gets old and doesnt
fit properly after some time. When you are driving (and steering left)
your right tyre will "spit" water exactly in direction of that hole.
Thats where the water comes in.

Seal it and your problems are gone. Hope it helps.