The headlights are controlled by the headlight relay that supplies power to the bulbs when the combination switch for the headlights is turned "On" to complete the ground side of the headlamp relay circuit. The relay is used to control a high current circuit in the same way a switch is used but it is more compact.so, the headlamp switch is a part of the power side of the circuit (but it switches the ground side of the headlight relay circuit); the headlamp bulb is the load and ground is through a ground wire from the bulb to the radiator support that holds the headlamp assembly. Heres the 'how the circuit works' from the Honda service manual:Low Beams (USA)The headlight relay receives battery voltage at alltimes. When you turn the headlight switch to theHEAD position with the dimmer switch in LO, groundis applied at the BLU/RED wire to the coil of theheadlight relay. The relay is then energized, applyingbattery voltage to the left and right high and lowbeam headlights through fuses 19 and 20. The lowbeam bulbs come on because the opposite terminalis tied to ground. The high beam bulbs and indicatorremain off because the ground path to these bulbs isinterrupted by the deactivated dimmer relay.Hope that tells you what you want to know.