24v 5 PIN change-over relay. A relay is used primarily to cut down voltage drop as power is taken directly from the main light switch to the headlights. The change-over relay should be positioned near the lights, or close to the fuse box. The relay is activated from the Main/Dipped beam switch. The main switch should also work through a relay.
The contacts are numbered 87 - 87a - 85 - 30 - 86. 85 and 86 power the relay through a switch and a 12v supply. 30 is common and typically connected to a 12v power supply. 87 or 87a carry the supply from 30 depending on the state of the relay (on or off)