Feel free to upload photos of the broken "rubber", I guess the screw is flying out, it is an M6X1 screw about 1cm long - see in the photo Mark (1).
Regarding the installation of the rear winker, to get an access, remove the lower cover, the one above the rear wheel. For removing you need to release two type A clamps (Trim clip A) - see in the photo Mark (2), push inside the clip central penis. To close, take the penis about two millimeters out of the clip surface, Insert the clip to the cover, and push the central penis to aligns with the surface.
Regarding the installation of the rear winker, to get an access, remove the lower cover, the one above the rear wheel. For removing you need to release two type A clamps (Trim clip A) - see in the photo Mark (2), push inside the clip central penis. To close, take the penis about two millimeters out of the clip surface, Insert the clip to the cover, and push the central penis to aligns with the surface.