Abstract: The invention contemplates an automatic headlamp switching system that enables an operator to use an existing vehicle headlamp switch to operate a secondary light source on an accessory unit attached to a vehicle. The vehicle includes a primary circuit having a primary light source, an energy source, and a parking/headlamp switch for connecting the energy source and the primary light source so that the primary light source is selectively energizable. A secondary circuit includes the secondary light source. A pair of relays are connectable in circuit with the primary and secondary light sources. When the accessory unit is electrically connected to the vehicle, the relays are operative to interrupt the primary circuit from the energy source to the primary light source and simultaneously complete the secondary circuit from the energy source to the secondary light source.