I had this problem with an old car. It was HELL!!!!
ok, Theres one definate solution which is funny as hell after a couple of drinks. When its dry, Sit in the car and get a friend to squirt the car with the hose! See if you can see where the leaks comin from. try the doors, sunroof-if you have one! and even the windscreen! Then you know where its comin from!
Apparently, water can get in through the rear and run through to the front... So see if the rear was wet too. If it is, it could be the bootlid, or seals around the rear light cluster.
Finally, The worst one. If you still cant figure it out, there is a chance the bulkhead may have a hole in it. (which is what happened to my Cinq....
) unfortunately, this would mean you'd have to take out the entire dash. fill the hole, or weld it. But thats a hell of a lot of work. When it happened to me, I part exed the car for the micra I have now!
... but kept the leak quiet
In the meantime, get a car-cover thing.. If you're worried
Hope that helps
Chris