ok - i had this same problem. Unfortunately, since the fiat has a slimmed down version of windows (version ce i think) it will not reconise the format that itunes (mac) use for playlists. For the car to see the playlists you need to use the windows playlist files, which are different to what file type that iTunes use. I was not going to stop using iTunes to organise my ipod.
Therefore i bought a 4gb flash drive and move the files over to it using my iTunes and making separate folders for my music and this stays in the USB port all the time (the downside for this is you can't play AAC files).
Probably not the best way by far, but the quickest way for me to have the music i want and the easiest way to organise it (for me anyway). I also bought the CD changer Ipod cable so the car thinks the ipod is the cd changer, and you label the playlists as 1 Music, 2 Dance music, etc... and each playlist will be a CD on the stereo display, ofcourse when you go over 10 playlists it gets a tad annoying to work through it all but it will play AAC files and any bought in the iTunes music store because it is playijng it off your Ipod.
Anyway, hope this helps.
Stu