The Intravee is an innovative piece of hardware that enables BMW’s of the previous generation to accept Ipods as sound source, and use the radio and steering controls to navigate through the Ipod’s structure. This Intravee enables E39 5-series, E46 3-series, the first model of X3’s and the first model of Z4 roadster and coupe owners to enjoy their Ipod in their car.
The Intravee is actually not an Ipod-adapter per-se: The Intravee only acts as an adapter between the BMW media protocol, and the AlpineNet protocol to transmit commands and track info back and forth. The actual Ipod-adapter is something such as the Alpine KCA-420i which enables an Ipod to talk AlpineNet and vice-versa. Keep in mind that the KCA-420i has limitations such as being only able to show up to 511 albums and 511 artists, which means that if you take your whole collection of music on the road, you might not be able to select ZZ Top as artist out of the menu.
Contrary to the other models which have cd changers and navigational computers mounted in the trunk compartment, the electronics of a Z4 roadster are placed in between the two seats, under the covering that runs out of the gearbox and drivetrain tunnel. This hampers the installation of the Intravee and the Alpine adapter, in such a way that large parts of the interior of a Z4 roadster need to be removed before access is gained to the existing cabling to enable the Ipod adapter.
As a sort of reference, I have gathered the Alpine CD Changer Installation documentation from BMW which describes how to install an Alpina CD Changer, including removing and restoring the interior parts that hide the electronics in between the seats. This gives access to the cables necessary to connect the Intravee and the Alpina KCA-420i adaptor as described in the Intravee Quick Install Guide.
Update: Another option is to access the necessary cables through the trunk, as explained in this thread on z4-forum.com