Lighting and/or signalling device of an automobile and assembly method of such a device

The device (1) has a light source mounted on a part e.g. case (2), of the device, and a light guide (7) mounted on another part e.g. shield (3), of the device. Guiding units e.g. ramps or inclined parts, and positioning units e.g. lugs and grooves, allow guiding of the light with respect to the light source, and position the light guide with respect to the light source during assembly of the parts. The parts include a support (5) on which the light source is mounted. The support and the former part or the latter part are connected by a sliding link.