I have seen more than several posts about folks wondering where certain wires go or what they do. I bought a signal (tone) generator similar to the one that telephone folks use. The idea is that one removes all power from the system, attach the signal generator to the wire, then one can follow it anywhere it leads to the parent switch, relay, etc. One can trace either way in a circuit: switch to the light, motor, relay or from the light (for instance) back to the switch.
I know all the Gurus already have one but there are some others that may not know about them.
Mango Mike: No, I don't remember where I got it--either Cornwall tools or Snap On