Low voltage wiring is always repairable. It is a tedious job however. Unless the wiring setup has changed a great deal. A wiring diagram for an S-6 should help alot.
You will need to use ONLY wire designed for locomotive use. GE and EMD have this. Not automotive wire.
I once read about some group that tried using auto wire and turned the locomotive into a giant toaster! Needless to say, they regretted it when the locomotive burned up !

Wire with Exane.

http://www.electrowire.com/exane-1069a1093/

http://www.electrowire.com/contact-us/

or here:

http://www.r-scc.com/catTransLocomotiveTOC.asp