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Sean Kiernan’s family kept a secret for decades. They had a celebrity in their garage.

Not a person, but the most famous Ford Mustang ever made. The green one from the 1968 Steve McQueen film "Bullitt."

More precisely, the one Steve McQueen drove. There were two lookalike vehicles used during production, a stunt car that got beat up filming the movie’s legendary chase scene -- which was reportedly sent to the wrecking yard after the film wrapped -- and Kiernan’s, the so called "hero car" that was always ready for McQueen’s close-ups. Everyone knew it survived and was sold, but its whereabouts had been a mystery for years.

That was until the Detroit Auto Show, where Kiernan rolled it out onstage to celebrate the 50th anniversary of the film and the launch of the 2018 Ford Mustang Bullitt, a limited edition tribute to the original.

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The original 'Bullitt' Mustang was unveiled alongside the 2018 tribute model at the Detroit Auto Show. (foxnews.1eye.us)

Sean’s dad Robert bought the car in 1974 from a classified ad in Road & Track for $6,000 and he and his wife used as a daily driver in New Jersey for the next few years. In 1977 Steve McQueen reached out offering to buy it back, but Robert told him "thanks, but no thanks."

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The clutch gave out in 1980 and Robert parked it in the garage with 65,000 miles on the odometer, planning to get it fixed someday. He never did. The busted Mustang then followed the family, which now included Sean and his older sister, through two moves, finally ending up near Nashville. Back then, before Internet car forums, a car like this could just disappear. And so it did.

But when rumors about the Mustang began spreading through the automotive world, and eventually through cyberspace, the Kiernans started to realize what a big deal it was, and they weren’t sure how to deal with it. They even kept the car’s history from family and friends.

Sean says they wanted to share it with the world, but it was partially disassembled at this point and they felt they at least needed to get it back together and running again first. Then he had kids of his own, and Robert came down with Parkinson's. He died in 2014.

Sean says he couldn’t go into the garage for some time after that, let alone work on the car. But with the anniversary looming, and inspiration from the few people he confided in, he finally got to running and reached out to Ford about unveiling it with the new car. Of course, this being a Hollywood story, there was a twist.

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Sean Kiernan at the wheel of the nearly all-original 'Bullitt' Mustang. (Ford)

Last fall, out of the blue, the stunt car turned up in a junkyard in Mexico. Then man that bought it, Hugo Sanchez, had no idea what he’d purchased until a records search revealed the rotted, repainted white Mustang’s past. "Bullitt" fever was in full swing and people started wondering again where the hero car was.

Soon it will be on the National Mall in Washington, D.C., where it will be on display after being entered into the Library of Congress’ National Historic Vehicle Register of significant American automobiles. After that, Sean is taking it on a tour of cars shows and events, and then loaning it to museums. The Henry Ford museum in Michigan has already said it has an open invitation to live there.

One place you won’t ever find it is in the classified ads, or on the auction block, despite being worth an estimated $5 million, at least. Sean said he will never sell it.

"It’s not worth more than my father, and the memory of him," Sean says. "You can’t put a price on that.

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