Right, I had the exact same problem over a year ago, and eventually managed to sort it after trying to find the source of the problem (I removed the whole dash, carpet, etc, etc, etc)........
It's kind of a design flaw really. Right, you need to do the following:
Remove the scuttle panel underneath the windscreen wipers, you will then have access to the interior air intake, etc.
What I found is that the seal around the clip holders for this scuttle panel had gone (specifically the clip holder directly above the hole for the interior air intake, on the passanger side of the car). As water ran down underneath the scuttle panel, it got through this clip hole (metal hole in the car bodywork), dripped directly down from here into the interior air intake, and dripped through the motor assembly and down into the passanger footwell.
I also found the water running along the air ducting from the motor/air intake section, and dripping down further along where there were foam seals between the plastic air duct sections (i.e. dripping down nearer the centre console). The water also found its way down behind the carpet from this hole, which meant that the carpet and foam underneath was soaked when I'd lifted it up!
I also found water coming through past the green windscreen level plastics (under the scuttle panel again, I had some green pieces of plastic at the bottom of the windscreen). The water would drip through the one above the air intake, and again, directly down into the car interior......
What I basically did was remove the problematic scuttle panel clip holder, and use roof flashing tape and a heat gun to properly seal the hole up. One clip removed wont affect the scuttle panel security. I also put a bit of flashing tape over the green windscreen clip, to seal the leak.
I had tried silicone previously, but the water still found its way through. Therefore I 'didnt' have to remove the dash (although I fitted the alarm at the same time), and you wont have to either!
Basically, remove the scuttle panel, pour some water onto the right hand side of the cars windscreen, let it run down and see if you can feel drips coming through underneath this metal section (stick your hand in the metal ducting). If you do, theres your problem! It'll drip directly down into the cars interior.
Mine has now been try/leak free for over 2 years.
Hope that helps
Edit: I thought it was the windscreen seal at first, but it isnt!