check the drain holes are clear, and remove the metal ecu cage. under this tyou'll see a square bit of metal thats boned down, this is covering the hole wherre fresh air blower inlet is on a LHD car, check this over carefully to make sure its not leaky.
another thing you can do, remove driver side dash undertray and lie in the footwell looking up, while a mate shines a high power torch around that area. any holes should become obvious!