The ornamental design for the truck bed cap wing, as shown and described.
FIG. 1 is a rear perspective view of a truck bed cap wing, in a state of use, showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a right elevational view thereof, the left side elevational view being a mirror image;
FIG. 5 is a front elevational view thereof; and,
FIG. 6 is a rear elevational view thereof.
The broken lines showing a vehicle in FIG. 1 illustrate environmental structure that forms no part of the claimed design. The broken lines on the surface or adjacent to the truck bed cap wing, illustrate portions of the article that form no part of the claimed design.