> There are devices that will tell you how long a cble is and can tell you
> > where shorts are. Pretty accurate, but expensive.
but it doesnt tell you where physically that cable break will be in the wall.
My usual method is to measure capacitance at each end of the lead. Its usually easy to see roughly where the problem is. Only works for open cables of course. But whether you use that or whatever, you dont actually know where the n% or 10' point is. So these kind of approaches are no use in your case.
Nail heads will be close to the surface. Drive your cable with ac, preferably a high audio tone, and use a very high input r opamp in a lil handheld box with a metal plate on one end. Connect metal plate to opamp input. Now run the thing along the cable route: at some point the nail to plate capacitance will max, theres your nail.
Regards, NT