water leak

If it fills sitting still - check that the drains in the quarter panel bottom pinch weld are open and draining (key tool - old wire coat hanger)

If it fills while you drive - I had a 1975 Dart that rotted out around the rear outside seat belt bolt (which comes out in the rear wheel well. The action of the tire slinging water drove the water into the car and filled up the back floor...and the front floor if you drove it long enough!!!