Headlight Ground(s)

Better do some tracing. I once had a used VW microbus with weak headlights. Both lights had a wire. Turned out that wire only connected the 2 lights together. The actual return path was a rusty body panel against a headlight nacelle. Once I added an actual wire ground I had actual lighting.

In older vehicles each HL would have a ground screw nearby but newer stuff has more weather tight connection.
So it could be either way but I would suspect that each light would have it's own ground circuit rather than having to run one wire large enough to carry the load.

Headlights take a lot of current. When vehicles leave the factory, they have no corrosion, and the lights work. One trick to getting brighter lights is to put a relay next to the light. The relays are cheap. You run a hot and ground from the battery, if possible, and use the existing headlight power to simply power the coil of the relay. You will get better power, and your headlight switch will only have to carry a small amount of current. You may already have a single relay somewhere, but one good hotwire from either headlight connector you have already, can yield you enough to control 1 or 2 separate relays for the headlights. The Bosch style relays are just a few bucks each with the plug connectors, and will attach anywhere with just one screw. Best part is the ease of troubleshooting after that. Don't forget an inline fuse for each one, sized for the wire that you are running......meaning, the fuse should melt before the wire.

Everybody says electric problems are usually ground problems, but I've found them being all sorts of things. To check a ground problem, just run jumper from the negative terminal of the battery to the light or whatever, temporarily. If it improves, you have a bad ground.

Well, it sounds like I'll have to run a test lead from the Negative side of the battery and try that first. Actually I might have to run 4 test leads! I cant imagine a separate ground for each of the 4 headlights, but then again there's a lot of things I can't imagine that are happening today! - Tom

No need to trace grounds. Just add additional ones.