The ornamental design for a reardecorativepanel for an automobile, as shown and described.
FIG. 1 is a perspective view of a rear decorative panel for an automobile showing my new design;
FIG. 2 is a front, elevational view thereof;
FIG. 3 is a top view thereof; and,
FIG. 4 is an enlarged front, elevational view thereof.
The claimed design is indicated by the relatively light portions in FIGS. 1-4, while the relatively dark portions are environmental and form no part of the claimed design.