If you have a 2006 or later, this is what you need. I just copied this off a site.
Note: If you have a 2006 or later model Prius, there is a AUX IN plug built into the car. No modification or extra equipment is needed. You can find it inside the middle console in the front seat. Just use your standard cable with a 1/8" mini-stereo plug on each end to connect it.
By the way, the 1/8" cord is the same cord as the 3.5mm to 3.5mm. You can but this at Radio Shack or any other electronic store for about 5-10 bucks.