Check the front door weatherstrip seals too.
On my '93 SSEi, the seal around the driver'* door was leaking along the front edge, and once water entered the cabin, it ran down the side of the floor pan, near the wiring harnesses, and ran downhill into the foot pan for the rear passenger seat.
Since the seals on the right side had been leaking too, but not as bad, I removed all 4 door seals, applied silicone sealant, and reinstalled them. They had all shrunk lengthwise, and probably should have been glued-on at the factory. After I reinstalled them, I filled the gap with silicone sealant and bonded a thin piece of black rubber tape on the outside surface.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles