What happens if you try the old filenames?
And just a general hint: the filename is a property that is needed by the underlying OS and is stored in the managing areas of the storage medium.
MP3tag deals with the metadata embedded in the files.
So if you can save the files on your storage medium and the OS does not complain, then there is nothing wrong with the filename.
It could still be that the format of the storage medium is not compatible with the car stereo - so check the requirements. Something to look for in the car stereo specification is NTFS or FAT.
If you have doubts that the files are actually MP3 files, see