The ornamental design for a taillight for a motorvehicle, as shown and described.
FIG. 1 is a front view of a taillight for a motor vehicle;
FIG. 2 is a left side view thereof;
FIG. 3 is a partially enlarged front view thereof;
FIG. 4 is a partially enlarged left side view thereof;
FIG. 5 is an enlarged first perspective view thereof; and,
FIG. 6 is an enlarged second perspective view thereof.
The broken lines depict portions of the taillight for a motor vehicle in which the design is embodied that are not considered part of the claimed design.