I claim the ornamental design for a spark plug wire retainer, as shown and described.
FIG. 1 shows a bottom, front side perspective view of a spark plug wire retainer showing my new design, the bottom, rear side perspective view being a mirror image;
FIG. 2 shows a top, front side perspective view thereof, the top, rear side perspective view being a mirror image; and,
FIG. 3 shows a top, front side perspective view of the spark plug wire retainer of FIGS. 1 and 2 mounted on an automotive distributor, with the unclaimed environmental structure of an automotive distributor being shown in broken lines. The broken lines showing an automotive distributor is for illustrative purposes only and forms no part of the claimed design.