Wiring system which comprises: a flexible printed circuit board (1) with a surface (2) in turn comprising one or more electrical circuits formed by conductive strips (4) and a plurality of electronic components (5) connected to said conductive strips (4) and with at least one extension or branch (6) extending directly from said surface (2) and also comprising conductive strips (7); characterized in that it further comprised: at least one flexible flat cable (8) joined to said at least one extension or branch (6), such that an electrical connection is formed between said flexible flat cable (8) and said extension or branch (6). A vehicle door which internally comprises this wiring system.