You need a 3.5mm plug on both ends - your local Radio Shack should have one.
I bought the Belkin Car in-Line Connection for iPod which is a retractable 3.5mm Cable from the Apple Store though its way pricey at $19.95 (hey I had gift card) - here's the link -
http://store.apple.com/1-800-MY-APPL...lm=TK879LL%2FA. Clearly one found on-line or at the Shack will be cheaper. Good Luck