Car audio has been a popular hobby and obsession since the early days of both cars and radios, and the head unit has evolved a lot throughout the years. They've gone from simple, monaural AM radios to complicated infotainment systems, and a number of technologies have come and gone over the intervening decades.
Most head units still include an AM tuner, but eight-track tapes, cassettes, and other technologies have faded into history. Other technologies, such as the compact disc, could also disappear over the next few years. That might seem far fetched, but the history of car radios is littered with abandoned technology that was once considered state of the art.