A multimeter records potential difference.
Your vehicle's chassis and grounds are kinda arbitrarily designated as 0 volts.
The battery positive terminal is also connected by a thick wire to the alternator. If we put the multimeter across the battery terminals, we might find that the positive terminal is something like 14 volts difference from the ground.
Now, if we measure the positive pin of the headlight socket, and compare it to ground, we might get like 12v.
A popular shortcut to this is to put one multimeter test probe on the battery positive, and the other on the positive headlight socket.
If the battery + is 14v above ground, and the headlight + is 12v above ground, then the difference between the two is 2v, which is the voltage drop of the harness. The multimeter will just read 2v.
However, IMO (and correct me if I'm wrong) an issue arises if they headlights are ground switched. Remember, any stretch of wire has a resistance, and will drop some voltage.
In a positive switched car, the relays and wiring switch the headlight positive, and the headlight grounds are generally going right into the chassis with a short stretch of wire and a screw, pretty low resistance, so it is often ignored. We assume the headlight ground is 0v, same as the car ground.
In a ground switch car, the relays and wiring switch the headlight ground, and the headlight positive is going straight to the battery (probably through a fusebox or something). In this situation, it is no longer applicable to ignore the voltage drop. In this case, the headlight ground might be actually 1-2v, since it has a pretty high resistance between the headlight socket and the vehicle ground, while the headlight positive might be 13.5v since there is less wiring between it and the battery.
Obviously, you seem like a guy who fixes things the right way, other you wouldn't be on CPF, you'd be on ebay waiting for your 8000k HID kit to ship alongside your blue-tinted brake lights. In that case, measure the voltage at the bulb (best idea is to do it when the car is warm and idling, and wait for it to stabilize to within like 0.1v so you don't have a bad reading because of a high idle or something), and measure it at the battery at the same time, then post the readings here.
Now, another important thing that not everyone realizes is that the bulb needs to be in the socket and turned on when you take the readings. If you stick your multimeter test probes into an empty socket, you'll find that there is only like a 0.1v drop over your stock wiring. That's because voltage drop is proportional to current. If there's no bulb, there's no current, and then there's no voltage drop.
