Well with the first CD players in cars you could only play CDs and once they began being able to read MP3 and FLAC files, then everything got interesting. However, the limited size of a disc which is about 750 MB, I want to say, had you hit a wall unless you splurged for a multi-disc change capable of reading MP3 files. There was no logical progression from CDs apart from DVD and BluRay and those aren't meant for vehicles. Bluetooth or connecting via a USB cable or outlet/inlet is the current preferred method, but I'm sure we'll see this change in the future.
Most of the cars I bought in the 90s were outfitted with a factory changer with 6 disc sleeves. And I'd carry a small album of assorted music on discs. Sounds so strange nowadays but so normal then.