My a/c system has been leaking freon. I had it checked out twice, no leakes in the engine compatment have been found. My mechanic says it must be the "heater box" that is leaking freon. Its apparently located under the dash of the car. He says it will cost approx $1000 to replace it. Does this sound correct? What exactly is a "heater box" and does this diognosis sound correct?

Thanks