I have just fitted a single headlight to my 2014 fatbob. New headlight has two wires, highbeam + lowbeam and earth is through casing. I have located highbeam and lowbeam in bikes wiring with test light but once it is all hooked up thd light does not work. DoI need to incorporate either of the left over wires from original loom. Thanks in advance, any help is very appreciated. 

Earth the black wire if it was earth to the old head light you will need to test it the light may be switched earth not power to turn on lights

Thanks for your advice perthhog. Do you mean I should earth original earth back to frame? I've put my test light onto the engine and touched other end to light bracket and it lights up. So I know power is getting to light. Just doesn't light up. I'm assuming it's something to do with going from double headlight to single. I'm going from 4 wires down to 2. I'm no auto electrician but I've wired up trailer lights and plugs and spotlights.stereos etc. But this has got me stumped. I'm so close.light looks great and I'm stalled by a wire.

Yay. I've got the light working. Only problem is that high beam is low and vice versa. Even when I swap the wires around. ...weird. anyone got a solution?

From the picture above, you are using the accessory wire (orange) to power up the light. Should be using the white (hi beam) and yellow (lo beam). The orange is for the park light which some aftermarket headlights don't have