Pauluk,
WOW, that was smart. I had to draw out a picture for that one. But sure enough, that works.
Here was my thinking:
on the roof: tie 3 of the 5 wires together. ground out one wire. leave the last wire open.
down in basement: identify ground wire as having continuity with ground. identify open wire as having no continuity. The other three wires will read continuity between them. tie the identified ground to one of the three remaining unidentified wires. and ground out another of the unidentified ground wires.
Back up on the roof: Identify which of the three wires is open, grounded, and tied to the original grounded wire.
Of course, a walk-talkie might work better.
