[Cam]

The tail lights are very easy to check. You have 2 access panels in the trunk directly in front of your tail lights. Remove them and check for moisture around the holes for the lights.

If there is moisture back here you will need to seal your tail lights with silicone. If the car was parked on a hill with the front facing downhill it could explain your wet seat, i suppose. But that could be an unrelated problem.