At the risk of being a pain in the neck, I will reiterate. The major sources of water in the rear floor pan are from three places. The hood hinge wells (or whatever they are called) are rusted out, the area under the grill around the windshield wipers is rusted out, or the battery box, which is always rusted out in these old cars. In your case the battery box is probably ruled out, since it is on the passenger side of the car.

The water from the hood hinge wells will pass under things in the front floor pan so you never see it. This is the most likely source of water in the rear floor pan. They are always rusted out, even in cities where there is no road salt, because people never clean the accumulated leaves out of them.

Richard