The singer leaves his home in Kentucky in 1949 to pursue work at General Motors in Detroit, Michigan. He installs wheels on Cadillacs, watching each one roll by day after day on the assembly line, knowing that he will never be able to afford one of his own. Beginning almost immediately, the singer and a co-worker decide to "steal" a Cadillac by way of using their assembly line jobs to obtain the parts via salami slicing. He takes the small parts home hidden in his unusually large lunch box; larger parts are smuggled out in his co-worker's motorhome. The process of accumulating all the necessary parts turns out to take at least 25 years, but once they have what they think is a complete car, they attempt to assemble the pieces. Because automakers inevitably make numerous changes to their models, designs and parts over the course of a quarter-century, the result was a hodgepodge of parts from different years and models that did not fit together well. Despite these problems, the singer and his co-worker get the car in proper working condition. The singer's wife is skeptical of the outcome, but wants a ride in it anyway. Townspeople begin laughing at the singer's unique car as he takes it to have it registered. However, the folks at the courthouse were not as pleased — it took the "whole staff" to type up the vehicle title, which ended up weighing 60 pounds. The song ends with a CB radio conversation between the singer and a truck driver inquiring about the "psychobilly Cadillac", in which the singer replies, "you might say I went right up to the factory and picked it up; it's cheaper that way". The song is in a moderate tempo in the key of F major, with a main chord pattern of F-B-C-F. The verses are done in a talking blues style; Cash had used a similar spoken-word format and chord progression in his earlier hit "A Boy Named Sue."
Bruce Fitzpatrick, owner of Abernathy Auto Parts and Hilltop Auto Salvage in Nashville, Tennessee, was asked by the promoters of the song to build the vehicle for international promotion. Fitzpatrick had all the models of Cadillacs mentioned in the song when it was released and built a car using the song as a model. The result was presented to Cash in April 1976. It was parked outside The House of Cash in Hendersonville, Tennessee, until someone could find a place to store it. After The House of Cash Museum closed, Bruce Fitzpatrick retrieved the '49–'70 Cadillac with a wrecker and brought it back to Abernathy Auto Parts and Hilltop Auto Salvage in Nashville, Tennessee, and crushed it.
The psychobilly Cadillac from the "One Piece at a Time" video is at the Storytellers museum in Bon Aqua, Tennessee. This is a different car from the one made by Bruce Fitzpatrick. This car was made at the behest of Bill Patch of Welch, Oklahoma, and constructed by Leland Mayfield, Harley Malone, Eldon McCoy and Don. P. Chenoweth.
An attempt at building a vehicle "one piece at a time" was completed successfully over a five-year period by a Chinese motorcycle assembly line worker in Chongqing.
