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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.