The Aux input cable is terminated in a set of RCA jacks. You'll need a cable that goes from RCA plugs to a standard headphone plug - these are readily available at any electronics store.
Then, you have two options. First, you could plug the headphone plug into the headphone port on your iPod (duh). Change the volume on your iPod, and the volume through your Elise's stereo changes too. Not the best situation, as this can lead to distortion if too loud, or, if the iPod is set too quiet, and you switch to FM, you be shocked by a blasting stereo.
A second option for connectin the iPod to the headphone jack is via the iPod's lineout connection. To use this connection, you need an adapter. I use the
imp. It has the iPod dock connector on one end, and a y-cable going to a lineout connection, and a connection going to a cigarette lighter plug for charging the iPod on the road.