well the unit i bought was alpine-specific and only works with semi-new alpines.
depending on your type of head unit there might be one, the unit is upwards of $100 new, but you definetly get what you pay for. i dont know how pioneer and others do it, but the alpine unit lets me sort through my ipod, change songs, play folders, all from the head units controls (all while charging the ipod)
if you're looking for a quick plug-and-play i believe there are rca-to-whatever that headphone jack is called. so you can probably just connect and it will play whatever you select on your ipod. (im not 100% you should probably ask a local (reputable) audio store.)
by the way, if you have an alpine, the unit is called KCA420i (youll see it named in the best buy fliers by the alpine units)
good luck
-Snipey
