Follow Gremlins link to Berlin BMaxís post, Iíve been tracking down a similar leak.
I park my car with the RH side on the kerb, on a slight downhill gradient, the water will leak in via the LH rear vent (behind the bumper), Soaks through the bit of sound deadening material in the corner cavity, seeps through the bit of sticky tape (ala B-Max-Mike above) runs along the metalwork and puddles in the LH rear footwell. If I park the car facing the other way, it runs into the spare wheel well.
When I first started investigating, I had water down the entire LH side footwell, front to back.
I stripped out the front seats and removed the carpet to dry it out, it was so waterlogged it weighed a ton! My first thought was that it was condensate from the AC, but after I had baled And dried out the footwell it started raining so I packed up for the evening, next morning there was another 1.5L of water in the footwell (this disproved the AC condensate theory).
I proceeded to strip all the rear interior trim and back seats out of the car, I could then identify the point of ingress ( through the tape) and follow the Stream along the side of the rear wheel well then down the sloped floor where the rear seat mounts and into the footwell at the rear.
Today I succeeded in removing the rear bumper, I removed the vents and cleaned and dried everything before applying silicone sealer and refitting the vents, Iíve refitted the rear lights and am now typing this, fingers crossed that if it rains tonight, all will be well (not that type of well) tomorrow morning.
A bit of a ramble I know but if it helps others........