Pull the battery and tray and look for a rust hole under there. It is quite common and then when the drain pockets are clogged the water spills over to the rust hole and runs down the floorpan to the back seat.

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'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."