Several times in the past, when troubleshooting strings of GI
lights, it turned out to be something as simple as a blown fuse.
However, when I first eyeballed the fuse, it didn't look as if
it was blown. There were no scorch marks inside the glass and
the fuse element (wire) didn't have any gaps. But, tracking the
open circuit back showed that the fuse was open internally,
somewhere. So, my recommendation is to check fuses for continuity
on those dead GI strings before spending time looking for problems
that aren't there.