My money is on the windscreen and bulkhead areas.
Sit in the car when it is hissing it down with rain, lift the carpet and watch. It may take some time to come through, hours in fact, but this will show you where it is coming in. If nothing happens, leak is coming through lower down ie. Lower door seal or from wheel arch area. Leaks are a pain in the ar*se. Had one in my mini. Rover forgot to seal the engine bay when it was built