Testing a wire
To test voltage on a wire that isn't bare, use a multimeter set to a 20V scale, take one of the probes, and poke it into the wire in question. You don't want to go all the way through the wire, just break through the insulation into the wire core. Take the other probe and touch it to a ground (negative battery terminal, frame, etc.). With the key off, there should be zero volts measured. Now turn the key on; there should be about 12 volts (a little less is fine). This means that the wire is only getting power when the bike is on, and this is what we want for this project. If this isn't the case. make sure the probes are where they should be and/or try another wire.
Here is some advice from Dan's Motorcycle Repair Course