The Z4 uses the same nav, audio and BT system as found in the E46, E39, MK1 X5 and MK1 X3.

It uses a BM54 tuner in the back for the FM and audio. You can google for BM54 Aux In.

The MK4 nav drive has DVD logo under the disc drawer.

There are many BT retrofit threads for E46 and E39 on Google. Only US cars came prewired. You will need the BT retrofit harness It will cost about £600 with used parts.

There are Parrot CK3000 E46 BT threads, and a new Connects2 BMW BT kit.

There is also the DICE Mediabridge and IntraveeII.