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The relay gets power direct from the battery. I cut the Yellow lead from the headlamp switch - which normally supplies all the power to the beam-switching relay - near the BS relay. Connected the relay output terminal to the BS end of the Yellow wire, the headlamp-switch-end of the Yellow to the relay's "trigger" terminal, and the relay's other trigger terminal to ground. Now the headlamp switch Yellow wire just turns the relay on, the relay then connects battery power to the Yellow wire into the BS relay. Now the headlamp switch carries ~3 Amps instead of the ~18 in stock form (with high beams on in 4-headlamp system).

This also has the advantage of increasing available voltage at the (brighter) headlamps, although on low beam they still use the bulb failure sensor circuit wiring.