After the deluge… any idea where this is coming from?

Hi learned folk; your advice & experience please…

I had had a leak in the back forever and finally sorted the rear door trim etc; and after lots of hose testing and many days of rain all appeared dry in the back… until yesterday’s storm; after which I’ve found this…

Headlining is bone dry, no drips or any evidence of any flow from anywhere… Everything else also bone dry. Is there some join in that wheel arch through which some ingress could happen?

Any ideas please? And on how to resolve/prevent?

Thanks in advance
Steve

looks like your seam sealant has failed ...