Water leak Water thoroughly soaked and pooled at rear floor

Water thoroughly soaked and pooled at rear floor on both passenger and driver side after two months sitting outdoors. I flushed the air conditioning hose, inspected seals on doors and windows which seems to be good. After drying the floors and driving in summer, it was dry for a while but now, there seems to be a small damp spot under the rear passenger seat, approximately a hands length in front door. Underneath The rear driver side seat is dry. FYI, We did change the window regulator on rear window behind driver seat. Any idea on cause of leak?