Blowing 20A Fuse to my Running Lights

Diagram shows the fuse marked "instrument feed" at one end of the box. That is all the little illumination bulbs on orange wires, inst' panel, shift indicator, ash tray, radio, etc... Remove that fuse temporarily will further isolate the park lamp circuit.
We know a short to ground beyond this fuse should blow this fuse. We don't know what a back feed into a orange wire from another circuit might do.
If/when you determine the fault is at a exterior lamp, the license plate lamp is one of them. Good hunting