I used the external FD cable running inside a plastic liner tube, and spiral wrapped the wire to the tail light

around the plastic tube ,

then hot melt glued the wire under the mudguard, until i was near the mudguard mounted tail light.

later removed it all when battery LED head & tail light power improved,
enough to make any short time of a trip extending into dusk in Higher latitude summers OK,
I'm fine with that ..

My Rohloff trekking bike, i use the shift cable housing to the external shift box,
then run the wire up the rack strut to the wired top, rack, mount tail light.