The ornamental design for a rearportion of a motor vehicle, as shown and described.
FIG. 1 is a right front perspective view of a rear portion of a motor vehicle showing our new design;
FIG. 2 is a right rear perspective view thereof;
FIG. 3 is a right side view thereof, the left side being a mirror image thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof; and,
FIG. 6 is a top plan view thereof.
The bottom view of the motor vehicle forms no part of the claimed design. The bottom view of the motor vehicle forms no part of the claimed design. The deemphasized and halftone portions shown in the drawings illustrate environmental structure for the rear portion of a motor vehicle that forms no part of the claimed design.