There are many causes but the result is the same. The lowest point in the interior is the rear floor. Any interior leak will cause the water to end up there. The water seeps under the coating and rots the steel from the inside out. There is a pinch weld running right through this region, and the water goes to the V and settles there.

You may have to remove some interior bits (floor mats and carpeting, lower dash cover) and have a friend try to incite a leak with a hose while you look around with a flashlight.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech