There are a number of possible culprits for water ingress. To name a few, sunroof drain tubes can block, water can drip through from the front compartment, door drain holes can get blocked, door window seals can let it through and as Julian says the plastic door liners can let you down. Most cars no longer have the liners fitted, it's always been a pain to refit them and as often as not a lazy person chooses not to replace them when refitting the door trim after a stereo upgrade etc. Shame really as they're relatively inexpensive from Porsche and can also be easily fabricated at home from a sheet of polythene.
My tip would be to get an able assistant to spray water on the car, concentrating on a certain area of the car for a while then moving round. From inside get your head under the dash and as many other inaccessible places as you can and keep an eye out for any drips. Once you've located the source(s) you can take remedial action.
]
