Not so much a question of being available, but being practical. The earlier radios were big, required a lot of electricity, and were not loud enough to be able to be heard while driving. There was also a problem with radios picking up the ignition noise. Here is an example of why radios didn't work so well at first, taking up lots of room. First practical one was the Crosley Roamio in 1931, and car radios become pretty common in the 1940's. Here's a site that will tell you perhaps more than you ever needed to know. The text is in French, but if you can't read French you will still find lots of ads, schematics etc. in English.