A lot of newer cars feature slick iPhone integration where you can control your music with just your built-in stereo, but older stereo systems don't have that same level of support. Instructables user cmonaco3 built his own controller that mounts straight onto the car's window.
Cmonaco3's project is a bit complicated, but the end result is a small remote control that displays track information and lets you skip through tracks. The build uses an Arduino, an LCD and a bunch of different connectors. It's a well-made little box, and Cmonoco's guide comes with everything you need to know. Head over to Instructables for the full guide.
If you're looking for a budget solution, don't forget you can always just mount your iPhone with office supplies. (But be cautious using it as a GPS; that's not legal in all Australian states if you don't have a commercial mount.)
iPod Information Screen [Instructables]