What was the first format of portable car audio?

Answer 1

im not sure what you mean by portable car audio,

The first car radio on record was fitted to the passenger door of a Ford Model T by 18-year-old George Frost, President of Lane High School radio Club, Chicago, and was in use by May 1922.

The first commercially produced car radio was the Philco Transitone, introduced by the Philadelphia Storage Battery Company in 1927. By 1933 there were claimed to be 100,000 cars fitted with radio in the USA.

The Marconi V2A (cabinet radio) was mounted on the running boards by some enterprising motorists in the early twenties.

...Car Radio In 1929, American Paul Galvin, the head of Galvin Manufacturing Corporation, invented the first car radio. The first car radios were not available from carmakers. Consumers had to purchase the radios separately. Galvin coined the name "Motorola" for the company's new products combining the idea of motion and radio.

In 1930 Galvin Manufacturing introduced this car radio as the "Motorola." It was one of the first commercially successful car radios, and the first major product for the company that later became Motorola, Inc. Paul V. Galvin created the Motorola brand from "motor" (motorcar) + "ola" (sound).

In Germany Blaupunkt fitted their first radio to a Studebaker in 1932 and in the United Kingdom Crossley offered a factory fitted wireless in their 10 hp models from 1933.

...In 1952 Blaupunkt became the first maker to offer FM receivers."

the first 8-track players were car decks that came out in the Fords in the fall of 1965

the first car record player was the Highway HIFI in 1956 Chrysler, DeSoto, Dodge, and Plymouth models--could handle 45-speed records as well as 7-inch records in the new 16-2/3 format. The players were installed on a slide-out turntable beneath the dash and hidden behind a drop-down door that could be opened at the push of a button.

the compact cassette player first came in automobiles 1975 by Pioneer

the CD player first came in automobiles in 1984 by Pioneer

and i think that should be a good start for you don't you think?