This is a low beam cutout harness for the GL1800 motorcycles. This is made by Electrical Connection and is designed to turn the low beam headlights on after the bike is running. Your GL1800 shuts the headlight off when you press the starter button. This process of cycling power (on with the key-off with the starter-on once running) is hard on both the bulb and especially on an HID ballast. This harness changes the trigger to your low beam relay so that the low beams only get power after the bikes engine is started.
Recommended: Installation under seat and trigger from oil pressure sender switch.