For the previous decade, audio engineers had been struggling with a way to make recorded music easy to use on the road; compact record players only worked while cars were stopped, and reel-to-reel tapes were impractical.
Building on the design of cartridges developed for radio stations, a group of companies led by Lear Jet and RCA came up with a plastic rectangle holding a single endless loop of tape, enough for eight songs, give or take.
Ford was the first automaker on this date to announce it would sell the new 8-track tape players as standard equipment in its 1966 model year vehicles; the format was so new customers would have to go to the Ford dealer to buy tapes.
Within a couple of years, the 8-track was the first truly portable music format, and served as the way to rock out to Boston and Three Dog Night on the road until the technically superior cassette surpassed it in the late 1970s. Here's Ford touting its innovation in a TV ad for the 1966 Mustang:
Ad for the 1966 Mustang: