I think the aerial is via exactly the same as for the normal "analogue" radios. On a 55 plate GTI you are likely to have the CANBUS gateway power drain issue, which only a newer CANBUS controller will fix (about £70 for the part but needs coding). You will need to do the same as people retrofitting RNS510 sat navs (but without the GPS antenna!), i.e. new CANBUS and an adaptor to plug the second generation aerial lead into the new radio. Search this forum and you will find threads where people have asked similar questions about retrofitting RNS510s and RCD510s into GTIs. Note, the firmware downgrade workaround for the RNS510 battery drain issue is not possible on the RCD510.