I think Bearbum wants to see which is the active wire in the headlamp loom so he can fit driving lights.
You don't really need to use a multimeter. Get a test light - you can just hook up a 12V light from an indicator - put one pole on the negative battery (black lead, exposed terminal, or just clamp it securely to the exposed metal on the motor). The other lead from the globe becomes your test lead. If you touch a wire in the car that's active with this lead, and the globe lights, you've found a live wire.
The trick with the headlamp is to figure which of those wires is live when the headlights are on and in high beam mode. Turn on the headlights, flick them to high beam, touch the three wires with the wire from this test lamp - only ONE of them will light the lamp, and that's the wire you need to tap into so you connect your relay for the driving lights.
So - test light is a small parking lamp/indicator globe that has two wires from it, one goes to any bare metal or the battery negative and the other is your magic test lead. Simple as that.
Test the light before you go hunting, just touch the test lead on the battery positive terminal to make sure the bulb works!
Hope that helps.