Today 35% of radio listening occurs in the car. Drive time is the airtime that every radioman cares about most. So, try to wrangle with the idea that radio predates the car radio by about thirty years.
Do yourself a favor and read the Arbitron study on in-car listening:
The cost of the first car radio was between $110 and $130, and was installed in most popular automobiles. The Galvin Corporation branded these products under the name Motorola, [MOTOR+ VICTROLA] It is not until 1947 that the Galvin Corporation changes their name to Motorola, Inc. http://www.motorola.com/
Battery-powered radios had been installed in cars by venturesome experimenters during the 1920s, but no commercial automobile radios were available. What Motorola did was to market a battery eliminator which converted the early, battery-powered radios to power line operation. In 1930, they put a working radio receiver into aStudebaker automobile and demonstrated it at the Radio Manufacturer's Association meeting in Atlantic City, NJ.
After World War II, factory installed radios were offered as an option, and by 1951 the automobile radio had achieved 50% market penetration.