Try feed across the fuse so you know if it before or after switch / fuse , the order is

1 battery
2 Ignition Switch
3 Light switch
4 Fuses
5 Bulbs

So by checking feed into fuse you know if the wiring is faulty between switch and fuse and as it is a single wire up to fuse this would seem to be your favorite , I have been known to simply join a length of wire between the 2 known good points ie cutting wire away from switch and into fuse and see if that sorts it