That would be the drivers side headlight plug. The extra wires go to the passenger side headlight. If you are wiring up a new harness, it would be a good time to run your headlights through relays and get a higher voltage(and brighter) to each light.
I'm not going to tell you your business, but maybe consider replacing the old headlight plugs instead of splicing into them. I know that it might take some time to get the proper parts but my feelings are with the old ends and connections to them there will be some voltage drop. It might not be anything that is noticeable but who knows. I look at using the old plugs like re-using an old pair of socks.
Just so you know the headlight plug with the single wire to each terminal is the end light on the circuit and the one with double wires are before it with one wire out of each plug going to the headlight switch while the other wire would go to the last headlight in the circuit.
As far as grounds, make good clean connections to the radiator support and then make sure that the radiator support and fenders are bolted together as well as to the firewall with good electrical connections between them all. I have seen issues with headlights grounded to the radiator support but then find out there is heavy paint buildup between the radiator support and fender and while mechanically solid and tight, electrically the connection can be poor.
Be aware too that adding headlight relays might not improve anything if the issues are with ground(s). Just because you have plenty of power getting to something like the headlights, if the ground is poor, the headlights could still be not as bright as they could be. This stuff all works in a loop.