A door module panel is initially mounted with a door harness and is provided with a panel hole at a position where a descending window glass does not interfere. The door harness is wired from a first surface facing a passenger compartment of the door module panel to a second surface facing a door frame through the panel hole. The door module panel has a side face, from a middle portion of which a protruding member is extended and bent towards the door frame. The protruding member includes a harness hole, through which is passed the door harness extending from the second surface of the door module panel. The door harness is then fitted into the harness hole with a grommet. The door frame is provided with a side panel notched with a groove. When the door module panel is incorporated into the door frame, the protruding member fits into the notched groove on the same plane. The door harness thus can be installed very easily.