If you have no luck "witching" for the wire, I would suggest seeing if you can rent a underground cable locater.
We have one and have good luck with it.
I have used it to find the far end of empty PVCs that had been installed during an earlier construction project but no one new where.
I pushed a 1/4" snake into the PVC and hooked the signal generator to that.
Worked great saved a lot of digging.
