I used the iPod2Car by Peripheral Electronics model PDXP. It uses different harnesses to connect to either the back of your factory head unit or in my case to the cable for the CD changer in the back of the car. You can skip, FF or Rew songs with the buttons on the head unit and it also charges your iPod as well. Newer iPods and iPhones need the latest model ipod2car and a special cable though.
I got mine
here for about $100 shipped but you can get them cheaper on ebay or so I hear.
There are some pretty lengthy threads over on CF about it as well.