I would start by placing the light switch to the PARKING LIGHT position, then use a test light or volt meter to see which wire has voltage. Then touch that wire to each of the headlamp wires to see which filament is lit. You can then mark the 3 headlamp wires for PARK, LOW, and HIGH. You may need to use a welding helmet to look at the headlamp bulb filaments. As mentioned, the low beam filament is on top (or at least it should be).