Q: Ive synced my library onto my ipod but it wont work in my car

Do you know if that USB connection works, when you plug in a regular USB flash drive that has MP3 files on it?  If it does, I think the iPod is just acting as a "data source" for song files (like a USB flash drive), and it is not actually playing the song (the stereo system is playing the song).  So playback settings on the iPod would not matter.

What may matter is the type of song file.  Maybe it can only play MP3 files, and your iPod has AAC files.  If that one song that did play was MP3, and the rest of your songs were AAC, that's why it was the only one that ever played, and just kept repeating.  Just a theory...

It may be helpful to know the specific iPod (nano) model, and the brand/model of your stereo system, in case someone reading this has direct experience.

AAC is the audio format that iTunes uses by default, and the format Apple uses for iTunes Store purchases.  If you ripped your music CD's into iTunes without changing settings in iTunes, you would get AAC files.

AAC is a standard format (like MP3), so most current digital music players can play it.  However, some cannot.

Do a Get Info (from a right-click) on a song in iTunes, it will state the format next to Kind on the Summary tab.

Im not sure what AAC is but can I change this type into an mp3?

You can. However , it is taking a compressed file and recompressing it in a different format. Not an ideal situation. If the songs are from CDs, you can reimport them as mp3

In Preferences/General you will see "import settings". Click on that and choose MP3.  When you imort from CD, it will now import as MP3.  If you select and right-click on an AAC song (or songs), the contextual menu will have "Create MP3 Version"

It may be easier to create a playlist in iTunes with the MP3 version of the songs you want on the iPod, then use automatic syncing to sync your iPod to that one playlist.  If you have been using the Manually manage music setting (you drag songs to the iPod in iTunes to load them), you can change to it to automatic syncing by setting up the Music tab for the iPod in iTunes.

Select the iPod in the iTunes sidebar under DEVICES; the Music tab is over to the right.  On the Music tab, set it up to sync that one playlist.

When you Apply the change, whatever is currently on your iPod will be replaced, based on the settings on the Music tab.

Yes.  In your main iTunes Music library listing, use the "plain" column list format (the one that does not show any album art).  In the column headings, see if you have one labeled "Kind."  If you do not, right-click anywhere on the columne headings and select Kind from the popup list (so that it has a check next to it).  It should now appear as one of your columns on the list.  You can drag the column heading to change column location.

Click on Kind in the heading, and it should sort the list by that column.  The songs that are MP3 should say "MPEG audio file" for Kind and be grouped together.  The ones that are AAC will say:

     AAC audio file <- songs ripped from CD

     Purchased AAC audio file <- songs purchased recently from the iTunes Store

     Protected AAC audio file <- songs purchased a while back from the iTunes Store

     (look for any other Kind with "AAC")

The key thing is that they will be grouped on the list by Kind, so you can easily select only the songs that need to be converted.  After a songs is converted, you will have a second listing of the same song in your iTunes library, so you definitely do NOT want to do this for your entire library. 

If your iTunes music library is large, you may want to create a new playlist in iTunes and only put the songs you want on the iPod on that playlist.  Then, do the conversion selection in the playlist, instead of in your main iTunes library.  After conversion is done, you can remove all the AAC versions from the playlist (they will still remain in your main library), so that everything is on the playlist is MP3.  Then, sync only that playlist to your iPod.

If you have lots of room on your drive you can do this for your entire library.  Personally, I ripped all my CDs into Apple Lossless format (which is about 10 times larger than an AAC or MP3) then made AAC versions of them for my iPod, iPhone, iPad, etc. Once in a while I have to make an MP3 version.  I figure that starting with Apple Lossless gives me the flexibility to create whatever versions I want later from the full-fidelity version, including new any new formats that may be developed.