From experience, the usual point of ingress is the window regulator plate seal. You'll need to remove the inner door card and you should see signs of water staining either around the lower part of the metal plate and / or the speaker. The reason it doesn't show up on the sill carpet is because the water sneaks past the door apature seal and under the plastic sill trim, then collects in the foot well.

The proper fix is to remove the whole regulator, remove and clean off the (frankly rubbish) foam seal strip and door then apply a good quality mastic sealant and refit.

You can get away with just removing the 10mm bolts that hold the regulator (leaving one slack in the top), prise the reg apart from the bottom and chock it out with a piece of wood or similar, clean off the seal strip as best you can and apply sealant around as far as you can then refit.

If there's signs of ingress from the speaker, you'll need to drill out the retaining rivets to remove it, apply sealant to the seating flange and rivet it back.

It's a good idea to leak test when you first remove the inner door card to be 100% sure it's the reg seal / speaker (which it usually is), then check after sealing the leak before re fitting.

Good luck with it.

Edited by The-Mechanic on 05/04/2014 at 12:14