The ornamental design for a rearportion of an automobile, as shown and described.
FIG. 1 is a rear, right perspective view of a rear portion of an automobile of our new design;
FIG. 2 is a front right perspective view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a right side elevation view thereof, the left side view being a mirror image thereof, with the exception of the fuel door, not found on the left side view; and,
FIG. 6 is a top plan view thereof.
The portion of the vehicle in light contrast is environment and is not a part of the claimed design.