It can be caused by:
1 ) AC drain being partially or completely clogged. Or, as has already been mentioned, the AC condensation accumulation section.
2 ) Heater core leak. If it's the heater core leaking it will be antifreeze, not water. And it will mostly be on the passengers side. You might temporally remove the cabin air filter and look in that area. Also, feal around in there to see if it's damp. If it is damp, it could be AC condensation or antifreeze.
3 ) One or more sun roof drains are clogged. There are 4 of them, 2 on front corners, 2 on back corners. You can clean them by pushing a wire or tube of proper diameter throug them. The back ones can be also cleaned like that by finding where they terminate. Most vehicles hide it behind a back wheel area trim piece.
4 ) Windshield leak. It might require a windshield installer to reseal it.
5 ) Very rarely, rain can get channeled in around a door.
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If you don't have a very hot dry environment to dry out the carpet and padding below it, you may have to pull the carpet and padding, dry it well, and put it back in. This is a lot of work, and takes quite a few hours.