You need to check all the seals, windows and door. If you can pull up the carpets, get someone with a hose on the outside whilst you sit on the inside. Water may be leaking down inside the trim, so you will have to remove the carpets to see exactly where it is getting in, as it should run straight down. The other possibility is a hose under the dash, or an air vent under the dash, sometimes you can get leaks from this area.