I`m pretty sure there are drain "slots" along the seam ? have a poke about with a screw driver. If no luck drill a hole,jack it up to drain the water all out then squirt loads of wax oil in.
I had the same problem with my 205gti ,it turned out to be the drian holes blocked up from new at the factory with sealer.look along the bottom edge of the sill their will be drain holes.