Sprint GT Wires.....

The horn will see you right.

 

http://www.instructables.com/id/Install-a-Switched-12v-Power-Outlet-onto-your-Moto/

 

Piggy back off the live terminal on your horn and then pick a suitable earth point for the earth wire.

 

To revive an old thread, I've just asked the same question and I think I now know the answer. If you look closely there should be two other similar wires nearby, mine were almost heading under the airbox. All four come from the Datatool alarm unit. I think the shorter pair are for adding an extra siren or a pager, and the longer pair maybe for adding an extra loop to make things like luggage/helmets more secure. In the early 90s the old Datatools had a long yellow wire with banana plugs on the end which did the same job!