Headlights are the largest power user on a motorcycle, and they're quite voltage sensitive; a couple of missing volts will degrade your lighting a lot.
The current will typically run through a fuse, the ignition switch, the headlight on/off switch, and the high/low switch. The wiring is usually barely sufficient when the bike is new.
After servicing many old vehicles, including bikes, I've found that those old switches lose between 0.1 and 0.5V each.
The solution is easy; headlight relays.
Just use the existing lines to power the relay coils instead of the headlight. Run a new wire, with a new good quality fuse holder, to the relays.one for highbeam, one for low.
Make sure the ground wire is robust and clean too, or better yet run a new one to the engine or battery negative.
Don't use crimp connectors, solder everything. It takes a little more time, but as I told my Portuguese neighbor while I was rewiring his 40 year old tractor; crimps, 1 or 2 years. Solder, 100 years.
You'll see a huge difference in light output with a relay. All cars have them now, and bikes really should.
I used to do a lot of riding at night, on unlit country roads. A 100 / 55 W headlight bulb was the difference between seeing and not seeing. Without a relay, the connectors would start to smoke after a minute.