Water leaking on rear floor , even when just parked.

Is there any water in the front floorboard? Are you sure it's not traveling from front to back? And does it ONLY happen when it rains? If it's solely a rain problem, the problem is likely in the cowling above the hood. It has either gotten clogged with debris and not allowing water to drain where it needs to, or it could be the cabin air filter area is clogged as well, which is right below the hood cowling behind the glove compartment area. Either can cause water to divert into the interior. It may be going into the front floorboard but pooling in the rear, especially if you park on an incline.