Unusual water leak-rear passenger floorboard full of water

[willrev]

After heavy rain I noticed that my rear passenger floorboard is full of water. I cannot for the life of me figure where it is coming in. Sunroof on the 82 300SD is dry. sidewall carpet under door jamb is dry. carpet on the floor is soaking wet and water is pooled up. Could it be coming from trunk?

I figure if it was the heater core that the front carpet would be wet. Does not seem to smell like antifreeze either. But man, it gets wet.

Any help greatly appreciated.