It is coming from the 3rd brake light seal. Had the same problem. My drivers side carpet was a puddle of water, I didn't think it was the brake light because nothing was wet behind the seat, but sure enough when I had my brother spray the hose on and around the brake light it started leaking down between the inside and outside back walls and over toward the drivers side then coming out and down the side under the door sills. You can either buy a new seal, new brake light, or put a bead of silicone around the seal. I bought a new LED brake light and it hasn't leaked since.