Hi RAC
maybe had same problem as yourself?? but hoping for a while, explaining the seatbelt rusting through dampness in my rear passenger footwell area.
After some advice from Mr Knight tracked problem down to rear window washer. Found, just today, this morning, that the leak was from dual-elbow joints in pipework for washer. Dual pipe joint positioned at roof height, immediatly above the upper rear passenger seat belt mount, at roof level.
On using rear wash, which you do more at this time of year, i detected fresh leaking fluid in the CD Autochanger area.
Think i've sorted now by removing one of elbows(loose 2nd elbow seemed redundant) and pannels to access joints pretty easy to remove, all pull off, no screws. Even headlining can be eased down slighlty a few inches using kitchen towel roll for easier access.
Good luck
Mark