Welcome to the site Ferg,
You should spend some time reading old threads under water leaks and see the problem areas and what other members have done to fix leaks. You might not get very many replies in an old thread about bet mat moisture.
Areas to check would be windshield seals,backed up a/c drain or sunroof drains, also door seals including the rubber boot that is visible when you open your doors in the lower hinge area, it should hide a loom of wires and be attached to door on one end and door frame on other, then depending on slope of parking you could have leaky midgate seals or rear window seal and the water is just running forward.....