The ornamental design for the light assembly for an automobile, as shown and described.
FIG. 1 is a front view of the light assembly for an automobile, showing my new design integrated into an automobile body; the automobile body does not form a part of the claimed design and it is shown for illustrative purposes only; a rear view of the light assembly for an automobile does not form a part of the claimed design;
FIG. 2 is a right-side perspective view thereof;
FIG. 3 is a left-side perspective view thereof; and,