I only had the car a couple of months but in the service records for 2005 there is a bill for replacing rear window because of a water leak ..Well they did not fined it and the water ran under the back seat and into the sill.
following tips from various threads I removed the rear window trim and in the bottom corner is an unwelded joint that had rotted through..see photo ..the yellow stuff is marine epoxy (24 hrs to set) mixed with flour to make a wet filler .
Signs of people being there before/ Outer rubber seal a bit damaged for about 1 ft so will fill with Silicone.

Thats not rust to the left just grot.