The ornamental design for a rearlamp for an automobile, as shown and described.
FIG. 1 is a perspective view of a rear lamp for an automobile showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a left side elevation view thereof.
The part of the photograph shown in a coarse dot pattern, along with the broken-line perimeter illustrates a portion of the rear lamp of an automobile that forms no part of the claimed design.