I'd encourage you to take a look at the hinge bolts on the passenger side when you lift up the hatch. I had the exact same issue a few months back and it was a result of not tightening one of the two or loosening after putting on the SURF hatch (see build thread if you're wondering). It was raining hard and I saw water dripping through and onto the mat. When I pulled over I lifted the hatch and saw where the water was entering. You'll have to remove the light cover and the plastic overhead piece but it's a piece of cake. I'm certain this is what you're experiencing. Cheers and let me know.
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