The ornamental design for a rearportion of an automobile, as shown and described.
FIG. 1 is a left rear perspective view of a rear portion of an automobile of our new design;
FIG. 2 is a left side elevation view thereof, the right side being a mirror image thereof, with the exception of the fuel door that is not present on the right side;
FIG. 3 is a front left perspective view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof; and,
FIG. 6 is a top plan view thereof.
The portions of the automobile in light shading are environment, and form no part of the claimed design. The bottom view of the rear portion of the automobile forms no part of the claimed design.