w123 rear passenger side footwell leak

[JEBalles]

I recently noticed the carpet in front of the rear passenger side seat in the footwell was wet. I waited a day and checked again and it was still wet. I took up the carpet and noticed the metal was wet. I sopped it up and left the carpet out to dry. I took a drive in the rain the next day and came home to find puddles where the carpet was. I figure there must be a leak somewhere, but nothing apparent. Does anyone have any advice?