Yeah, I can confirm now that it was an underhood trailer running lights relay fuse. (not a relay -- fuse to a relay)
On a late F350 it's in space #26, it's 30amps (pink)and is a shape I was unfamiliar with. Basically a 1/2" cube with a translucent lid so you can see the traces between a pair of solder dots. The local autoparts store had it. This one is called low profile as there is a similar one 50% longer. No idea why.
Now I just hope the damaged section of wire I replaced is the only fault.