Run a search for the following:
water ingress
battery plenum
sunroof drains
The typical spots for a leak are under the battery (easy fix), sunroof drains clogging with debris (more difficult), under the cabin air intake filer (pita), under the ECU box (pita), and then of course the very uncommon spots that could be anywhere. I suggests you first check under the battery for pooling water, if you find it, poke the drains and remove the debris and you're done. If that is not the issue, open the sunroof and look at each corner of the tray for organic debris, the rear drains are quite far back as the tray extends several feet behind the rear edge of the glass. The felt liner that surrounds the glass is designed to allow water to pass through, it is not supposed to be watertight (hence the four drains).
If you look at all of these and find nothing, then remove the kick panels in the front footwells (quite easy) and sit in the car while someone sprays the windshield with a hose, and watch for water. The searches will provide more information about these fixes. Good luck, water ingress is a PITA.
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