Most head units now come with a direct iPod/iPhone connection.
With an iPod lead the lead is connected into a port at either the front or rear of the head unit, with the iPod connected at the other end all of the music stored on the iPod can be played through the head unit, with full control from the head unit, there is no no need to touch the iPod.
Many iPod leads also include video iPod for playback of iPod videos directly onto a multimedia unit.
OEM iPod adaptors are ideal for adding iPod connectivity to factory fit head units, the connection is via the CD changer port.
OEM adaptors are available for a variety of makes and model of vehicle.