I had this problem with my right-hand drive 147. Whenever it rained (and, living in Scotland, that means pretty much always) water gathered in the driver's footwell. The carpet was wringing wet. There were signs that water was getting in somewhere around the bonnet release catch, and the reverse side of the dash moulding was often wet if I ran my finger along it. The leak was unaffected whether I had driven the car or not. My local Alfa garage took three days trying all the obvious things (door seals, mostly) and optimistically returned the car to me, but unfortunately I was still able to drive and grow cress at the same time - so it had to go back to them. They eventually traced the leak to the "A" pillar and had to take the offside wing off to fix it, but they did fix it at this second attempt - which was just as well, because the whole thing took the best part of 2 weeks. Anyway, now I am completely dry (those elasticated legs are marvellous) and my only souvenir of this episode is a white watermark on the carpet.