It was one of the biggest mysteries in both Hollywood and automotive history: What happened to the iconic green Ford Mustang from the Steve McQueen film “Bullitt”?

There were actually two cars used during the making of the 1968 detective thriller, one for stunts and the other for McQueen’s close-ups. The stunt car, which was wrecked during the film’s legendary chase scene, was believed to have been sent to the scrap heap, while the hero car was simply sold and disappeared from the public eye.

(Warner Bros.)

Flash forward to 2017, when a car restorer bought the shell of a Mustang he found in a junkyard in Mexico and brought it back to his garage. There, he and a colleague discovered some odd modifications, and research confirmed it was the stunt car from “Bullitt.” How it ended up in Mexico remains unknown.

Then, at an event the night before the 2018 Detroit Auto Show, where Ford was unveiling a new “Bullitt” tribute edition of the latest Mustang to celebrate the 50th anniversary of the film, the hero car rolled out on stage next to it, shocking those in attendance.

Turns out it had been owned by the same family since 1974, when a man named Robert Kiernan bought it from a Road and Track classified ad for just $6,000. His wife used it as a daily driver until the clutch broke in 1980 and they parked it in the garage with plans to repair it that never came to fruition. As the car sat around deteriorating, its legend started to grow. In time, the Kiernans realized they had something special, but weren’t ready for the attention they knew it would get if they went public, so they kept it secret from all but a few people.

(Ford)

Robert died from Parkinson’s in 2014, and for a long time afterward, his son Sean couldn’t even go into the garage to look at the car. But with the encouragement of those who knew about it and the anniversary looming, he finally decided to get it running again and share it with the world. The otherwise unrestored car has been on tour ever since, with stops at car shows, museums and even a display on the National Mall in Washington, D.C. At the beginning of the journey, Sean never dreamed he’d ever sell it, but now he’s had a change of heart and is putting it up for auction.

“Everything I’ve done with it takes so much,” he told Fox News Autos. “I’ve got a baby on the way, third child, and it’s time for the next chapter in our lives.”

(Ford)

Sean said he has thought long and prayed hard over the decision, and even lent it to LeMay – America’s Car Museum in Tacoma, Wash., for a few months in part to see if he could live without it. But ultimately, he asked himself “what would dad do?”

“He’d say let it go. It’s worth a life-changing amount of money that would secure the future for your family.”

Exactly how much money?

Hagerty vice president of public relations Jonathan Klinger says his experts are sure it’ll garner the highest price ever paid for a Mustang, and $5 million wouldn’t surprise them. But it could go for much more than that.

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“How do you place a value on a car that is associated with Steve McQueen -- who is one of the few Hollywood celebrities that impacts values of cars -- but also is associated with one of the great car chase scenes, was in hiding for nearly four decades and is in unrestored condition?”

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The answer will come at the Mecum Auctions event in Kissimmee, Fla., in January, where the Mustang will cross the block with no reserve. Kiernan said he’d received a lot of offers to buy or broker it over the past year and a half, but decided to go with Mecum for one particular reason.

“When I met [owner] Dana Mecum, he struck me as the kind of guy my dad would’ve wanted to talk cars with over a beer.”

When it's all over, Sean will still have a green Mustang in his garage. As part of the thanks for helping it out in Detroit, Ford gave him the second 2019 Mustang GT Bullitt off the line.

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