moisture - 1991 300 wagon - rear door pillar

I have discovered a moldy section of carpet extending from the rear door pillar downwards both into the rear door well and the back cargo area in
my 300 Wagon. I am puzzled where this is coming from, the seals seem good on the windows (though I am no expert), where else could the moisture be getting in? Sunroof and along the headliner? roof rack? window seal?
Any methodical trouble shooting tips for tracking this down? Cheers
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