If you have an 04+ big dog DO NOT use a relay wired directly to the hand control buttons. Relays dont have a diode reversed across the poles of the coil inside. When you energize the coil, it stores electricity, when you release the button, without a diode it will fire back through the button contacts like a set of points. A 15+ amp relay is also too much draw for the buttons, a relay driver is needed. You only have 100ma of capacity at the button at 12v since they are rated for 5v 250ma.
If using an AR284 (vw latching relay) a driver is absolutely needed, and you need to cut off the point reed inside the relay that powers the flash to pass circuit inside. If you dont you will feed the headlight directly through the button when switching to low beam.