The 2006 Prius has an "Auxiliary" port, and 12 Volt outlet, located in the front seat console armrest that allows for MP3/WMA playback capability. With the Stereo is switched to AUX, the IPOD will play via the Prius speakers.
The AUX port is NOT and intergraded interface for the IPOD - you will not be able to switch tracks from the steering wheel or MFD. You will, however, be able to change the volume using the MFD or steering wheel controls.
The IPOD (or another similar device) is connected via the headphone jack to and the Prius using, what is basically, a cable with a head phone jack on each end.
Here is an example of the cable used:
