The ornamental design for a rearportion for a motor vehicle, as shown and described.
FIG. 1 is a front left perspective view of a rear portion for a motor vehicle showing our new design;
FIG. 2 is a rear left perspective view thereof;
FIG. 3 is a front right perspective view thereof;
FIG. 4 is a rear right perspective view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a left side view thereof
FIG. 8 is a right side view thereof; and,
FIG. 9 is a top plan view thereof.
The bottom view forms no part of the claimed design. The de-emphasized and half-tone portions shown in the photographs illustrate environmental structure for the rear portion for a motor vehicle that forms no part of the claimed design.