This '49 Merc would have been a hot rodding icon anyway, but Hollywood made it a cultural icon as well.
There is something almost impossibly cool about the 1949-1951 Mercury Eight Coupe. So many years after the release of "Rebel Without a Cause", it is difficult to tell whether the car's current mystique is due to the movie's success or the car's popularity with hot rodders, or indeed the relation of these two things to each other. Most likely it was a combined effect, with the film bringing more attention to a car already very popular with hot rodders.
Rebel Without a Cause, along with "The Wild One" a couple of years earlier, are some of the oddest cultural artifacts to have come out during this time in American history. Eisenhower's America was a profoundly socially conservative era, yet here were two films that glamorized rebelliousness so effectively that the protagonists still look cool today. Hunter S. Thompson's book "Hell's Angels" even reveals that The Wild One had had a profound effect on many early Angels and their desire to become outlaw bikers. In 1955, Rebel Without a Cause brought this rebellious behavior into an American high school.
Jim Stark (the protagonist, played by James Dean) wasn't someone who had grown bitter with age, and neither was he some two-dimensional villain. He was a relatable young character who just couldn't follow the rules. The Eight model was unveiled in 1938 for the 1939 MY, and was the first vehicle from the new Mercury marque. It would remain the brand's only model until 1952, and pre-'52 cars are often simply referred to as Mercury, leaving out the redundancy of a model name. The Eight slotted into the lineup between the cheaper Fords and the pricier Lincolns, and was already a hit before the outbreak of WWII.
Following the war, the Eight got a redesign for 1949, a time when most automakers were still putting out mildly updated prewar designs. This gave the Eight an edge over its competitors, and both Ford and Mercury set sales records that year. The design would look pretty dated after a relatively short time, but it was an important first prewar step for Mercury, and features like an option of one of the most technologically advanced car radios made at the time helped to make the '49 Merc a standout. It was as early as 1949 when Sam Barris would use what was then a new Eight to create the first of what hot rodders call a "lead sled".
These cars are so called because lead (and later Bondo) was used to fill in spaces left by the removal of exterior moldings. These cars emphasized style over speed, and the menacing look of a chop-top '49 Merc made them a car to receive this kind of treatment. This was in contrast to the treatment received by the "Deuce" (a '32 Ford Model B coupe) which was popular with hot rodders for going fast, and was usually lightened and given more power. Those who preferred the Deuce tended to look down on those more concerned with style, and the term "lead sled" was actually originally meant to be insulting.
But in 1953, Sam and George Barris created the Hirohata Merc, a chopped and slammed 1951 Mercury Eight Club Coupe which became an instant public sensation. The coolness of the car was undeniable, and such cruisers gained a certain degree of acceptability. All of this set the stage for the decision to use a '49 Merc as Jim Stark's car in Rebel Without a Cause. It was a good looking, but also slightly subversive car to drive, and this worked well with the character. Add to this the fact that the movie was released just a month after Dean's death in a car crash, and the mystique of the film and its elements were thereby elevated.