These head unit (radio) adapter units from Peripheral, and those from another manufacturer called P.I.E. are said to work very well with iPods and iPhones. However, a handful of different models are now available so make sure you select the correct adapter for your vehicle and iPod/iPhone combination. The newest adapters use a single, direct-connect cable with the newest vehicles and allow you to control the newest iPods and the original or 3G iPhone using the vehicle's radio or steering wheel controls -- and supports charging at the required 5 volts (also used for USB).
But first-generation Peripheral and P.I.E. direct-connect adapters may have 12-, 13-, or 14-pin connectors and thus, only control older iPods that support charging at 12 volts (like Firewire). This means that a given direct-connect Peripheral or P.I.E. adapter may or may not work with the newest iPods and iPhones. In those instances, you have to connect to the iPod/iPhone using a Peripheral or P.I.E. AUX adapter AND an RCA-to-3.5mm (mini jack) audio cable. This means you won't have radio or steering wheel controls, and you'll have to operate the iPod or iPhone directly. But this approach also does work when using iPods and iPhones with some of the older vehicles...Any of these provides far better connectivity and audio quality than a cassette tape or FM adapter.