Could you elaborate on how you determined the short circuit is in the wiring harness? Did a fuse blow? Did you disconnect the fuel pump and aux air valve when measuring for the short?

To answer you questions:
One: try to isolate as much of the wiring as possible by unplugging connectors, if you haven't already (and as suggested by Ron).
A good short circuit to ground will be hard to trace with a multimeter, the resistance is just too low and the resolution of the meter too poor to give you relevant readings when you get closer to the short.
Two: yes, but only if that wire is exclusively going between the aux air valve and the fuel pump, meaning no other circuits are connected to that wire in between.