Steve McQueen racing through the hilly streets of San Francisco in his Mustang GT fastback will go down as the most daring and iconic car chase in Hollywood history. The nine-minute scene in “Bullitt” took four weeks to shoot, cemented McQueen’s status as the “King of Cool” and made a star of the Ford Mustang.

Car enthusiasts always wondered what happened to McQueen’s dark highland green Mustang. Two were used in the 1968 movie, and one, a heavily-damaged stunt car, was discovered in a junkyard in Mexico in early 2017. The other, the so-called “hero” car, seemed to have vanished. Warner Bros. had sold the car to a private buyer post-production but it quietly appeared again in a 1974 Road & Track classified ad.

PHOTO: Steve McQueen is pictured in a scene from 'Bullitt.' (Silver Screen Collection/Getty Images) More

“1968 ‘Bullett’ [sic] Mustang driven by Steve McQueen in the movie ‘Bullett’. British Racing Green, 4 speed, new tires, 19,000 miles, mint condition, can be documented. Best offer,” the ad, printed near the bottom of page 121 in the magazine’s October 1974 issue, read.

PHOTO: Robert Kiernan was the only person to respond to the Mustang ad. He purchased the car for $6,000. ( Ford) More

Only one person responded to the ad: 26-year-old Robert Kiernan of Madison, New Jersey. He paid $6,000 to a man named Frank, who happened to live just an hour away from the Kiernans.

“My father and mother had seen ‘Bullitt’ and knew about the car,” their son, Sean Kiernan, told ABC News. “My dad was definitely a car guy.”

Robert Kiernan, a fan of classic cars, had already owned a 1966 Mustang, a 1970 Alfa Romeo Spider and a 1973 MGB GT.

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Robert Kiernan and his wife, Robbie Kiernan, would take their new Mustang for weekend trips to Maine and Upstate New York. It was also the car that Robbie would drive everyday to St. Vincent’s Parish, where she worked as a third-grade teacher.

PHOTO: Robbie Kiernan shown with the Mustang in 1977 in Upstate New York. (Sean Kiernan) More

“The body was not in the best of shape but it was a daily driver,” Sean Kiernan, 36, said. “We never referred to it as the ‘Bullitt Mustang.’ Some people were even skeptical that it was the same car from the movie.”

The Kiernans chose to leave the Mustang in the same condition that McQueen had driven it: no headliner or sound-proofing installation (both were removed for the movie), a straight pipe exhaust (to improve performance) and gaffer tape on the rear seat belts. Even the car’s paint is original.

“The car didn’t impress me much back then,” Kelly Cotton, Sean’s sister, told the Detroit Free Press this month. “You could hear the car before you saw it. And there were holes in the floorboard on the passenger side where the camera mount was used for the movie. I could watch down at the road as we drove along.”

PHOTO: Robert Kiernan purchased the Highland Green '68 Mustang GT fastback in 1974 for $6,000. (Ford Motors) More

McQueen must have sensed that his former partner-in-crime was living out its life in suburbia. He called the Kiernans a few years after they purchased it, asking for his car back. The answer was no, according to Sean Kiernan. A week later a letter arrived in the mail from McQueen.

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