FIG. 1 is a perspective view of an auto stereo mounting device, showing my design; FIG. 2 is a front elevation view of FIG. 1; FIG. 3 is a rear elevation view of FIG. 1; FIG. 4 is a right side elevation view of FIG. 1; FIG. 5 is a left side elevation view of FIG. 1; FIG. 6 is a top plan view of FIG. 1; and, FIG. 7 is a bottom plan view of FIG. 1. The broken lines shown in all views illustrate mounting features which form no part of the claimed design.