It’s nothing to do with alphabetical order and little to do with copying them to the USB in sequence, sub-directories don’t help so what’s the secret?
Your car radio reads files from the USB in the physical order it finds them on the disk. This order gets more and more disorganised as you add & delete mp3 files. So here’s what you have to do:-
Copy any MP3 files you want to keep to your PC. Then REFORMAT your USB Stick, it’s best to do full reformatting rather than quick. Now copy your MP3 files back to the USB Stick in the correct order. It may be possible to do this with Windows Explorer but I don’t take the risk I write a little batch file to copy them one by one.
In future avoid deleting individual files on your USB stick delete the lot each time.
(It nearly drove me crazy before I found the solution)