fogged rear window?

If he can whip it away on the inside it is leak somewhere. Dry everything up, let him get inside the car and let someone run a hose over the back glass starting around the lowest points working up until the leak is found. Thats how we find leaks at work (auto glass).

The leak can be fixed with window bed a glaze most of the time if it leaking round the back glass weatherstripping . The key is to kit the glaze under the weatherstripping. It will be messy but after it is cleaned up it will be sealed.