The ornamental design for a rearlight for a motor vehicle, as shown and described.
FIG. 1 is a first perspective view of a rear light for a motor vehicle;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a third perspective view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a top view thereof.
The broken lines depict portions of the rear light for a motor vehicle in which the design is embodied that are not considered part of the claimed design.