the cheap way is to connect the ipod using this type of cable to connect to the front of the HU.. (connects place for earphones to stereos 3.5mm input if there is one on the front)
Walmart.com: Accell 7' 3.5mm To 3.5mm Stereo Audio Cable For iPod: iPods & MP3 Players
in order to control from the HU rather than the ipod itself you'll have to connect to the back of the HU. use a cable such as the one below. check out my DIY if you want to bring the cable into the center console. others have ran the cable to the glovebox or cubby above the HU
Kenwood - A/V Cable for AppleŽ iPodŽ and iPhoneŽ - Black - KCA-IP302