My friend had an issue on his mk5 where the spare tyre well/carpet and drivers side footwell was soaked, this caused the windows to steam up all the time and on some mornings even have water droplets form on the inside of the windscreen
The spare tyre well was pretty straight forward, the vent behind rear bumper had perished seals and was letting rain water in due to this, we replaced the vent and problem solved
The footwell was the tedious one, the door seals were first suspects as they are adhesive without clips (very poor design) and were coming away in quite a few places, these were glued back in place but did not cure the problem, we then checked the door closure and found the door catch could do with adjusting so door closes tighter which although made an improvement did not cure the problem.
Finally it was time to water test the problem, so out came the door card and water was poured onto the door frame and window whilst i sat inside, low and behold there was water trickling in from a wire clip which has a sponge seal of some sort and is lower down the door so i assume when the rain/water from windows gather in the door itself it leaks out before draining at the bottom. This was cured by simply filling the clip hole with silicone sealant and pushing the clip back in which formed a water tight seal, water tested again and no more leaking.
The water was tricking down and finding its way into the lip of the rubber seal on the door frame, if enough water gathers in the seal lip it will flow down in to the footwell soaking the sound proofing and the carpet.
Although not needed doing on my car, i thought it will help cure this common problem. Its the green clip on bottom left of pic that was leaking on mates car, however other causes could be the blue clip or black rubber grommit
