I had the same issue. I tracked it down to a plugged sunroof drain. The sunroof overflowed, dripped along the drain pipe and landed in the footwell, behind the fuse box.

The sunroof drains are one at each front corner of the sunroof, just where the wind breaker lowers into when the sunroof is closed. The holes are about 1/4" in diameter, very easy to clog.

Nightmare to fix ... had to remove the carpet and get it cleaned as it has a waterproof layer that prevents from getting at the moisture under the carpet.

It's a big job. Take out all the seats, center console, bottom half of the dash, A and B pillar covers.

While I had the carpet out, I also noticed the sound proofing tar had all cracked and separated from the chassis, so I had to replace all that too. I have some pictures if anybody is interested.

Good luck!