Glad you got it figured out! The likely reason for your troubles is that your flash drive was probably formatted as "macOS Extended (Journaled)" file system, which is actually HFS+ file system, and therefore cannot be read by the in-car infotainment system. Conversely, the Mac can read and write to FAT/NTFS file systems. As an FYI: your files themselves didn't change, just the format of the file system of the actual flash drive.