Id be willing to bet its the White/yellow cable, next to the Earth on the stereo power connector. My car has had something connected to this and the earth cable, as there is black tape covering a section of removed insulation on each cable. However, i just tested this cable and could find no difference when the stereo is muted or not. My car was owned by a Citroen UK Sales Exec and has no built in bluetooth, so Im thinking this was possibly for a phone kit.