I suggest you verify the voltage drop between Battery and voltage regulator, both the black wire connection and the green, before messing with the regulator.
Stay off the "adjuster" as most don't understand what it does or how set it properly. It is to set the max limit of voltage allowed at the battery, where it then tells the alternator to back off a bit so the battery can't "boil". It is critical that the Vreg get an accurate report of true battery status, or it can't do it's job. It doesn't really regulate system voltage, other than to keep the battery healthy. That is the system regulation, battery status.
Ign switch contacts can become marginal and vibration can make them worse. This will drop voltage to the black wires, as can fuse box and other system connections and it can vary with the load (lighting placed upon it). Basic ohms law. E=IxR Increase resistance or current, the voltage reduces.
The switch and connector losses can make headlights dimmer, as less voltage to headlight makes that dimming inevitable. Drawing more current in a faulty distribution system will just make dimming worse.
You can easily check for distribution losses directly with a voltmeter. One probe on Battery POS the other on the vreg black. The loss will display directly. Turn lighting on and off and you can see more loss with lighting on. Don't forget the green to Batt NEG path, as it can have losses also, load dependent. Sum the worst case measurement, and if you see more than 0.5V loss, something needs repair.
Cheers,