The ornamental design for a rear endportion of an automobile, as shown and described.
FIG. 1 is a perspective view of a rear end portion of an automobile showing my new design;
FIG. 2 is a side elevation view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a front elevation view thereof.
The portion of the automobile shown in broken lines throughout the drawing views are for illustrative purposes only and form no part of the claim design.