Hmmm, the player is supposed to be able to play MP2 files directly according to this FAQ entry.

However, keep in mind that "MP2" is just a file extension, it's just three arbitrary letters. Perhaps there's more than one file format which uses that extension, and the car player only plays one of those formats.

For instance, maybe the car player likes to play "mpeg1-layer-2" files, and what you've got is an "mpeg2" file. Or something like that.

Can any of the empeg guys chime in on exactly what kind of MP2 file the player is supposed to play?

In any case, your temporary workaround will need to be transcoding the file to MP3 as suggested in the FAQ entry linked above.