Jochen has everything covered, for the absolute minimum you need

Nav computer (mk3 or mk4, this is for the navigation option only)
Radio module (this is to provide the radio AND all the audio amplification for the system)
Screen (4:3 or 19:9)
GPS antenna
SatNav Loom

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.