[HoodStrype]

Hmmm... if it does smoke when the heat is shut off, I don't know what that could be. It maybe time for another heater core?
In any event, you've had the rear seal replaced twice and it still leaks. I would suspect a crack. You would think if that was the case the trained eye of a mechanic would have caught it. If it's the same guy he could have the wrong part twice. Can you put a rear tranny seal in backwards? I would ask to see one at a parts store. Jack that car up, put a couple of bricks or chocks at the front wheel and throw her on some jack stands. Then get under there with a flashlight and look at it up close. Clean it real well, drive it around the block, and see where exactly it's leaking. Alot of that wetness could be left over from the first leak as well.