IPod shuffle 3rd gen/gmc audio system

[sllls962]

I have a 3rd generation IPod shuffle, which contains music from ITunes, and three songs I downloaded. When I downloaded them, I brought them into my ITunes library, and converted them to the proper format.

We just purchased a 2010 buick, with a USB plug to access the audio system. I plug my shuffle in, it recognizes it, but only plays the three songs I downloaded. It won't play anything, or even recognize any of the songs I purchased through Itunes.

I did go in, and convert the file formats to MP3s, but that didn't help. Interestingly, my son has NANO, and it plays everything on that with no problem. Normally I would just think my little shuffle is too old, but since it plays a couple of songs, that can't be it.

I've looked in the owners manual, and talked to several apple users I know. I'm stumped, so if anyone has any ideas I would appreciate it.

Thanks so much!