The tail light has two circuits for the ignition key switch. In park position, it sends power to the brown wire, which goes straight to the tail light filament. In regular on position, it sends power through the brown red wire, where it is soldered to the brown white wire somewhere in the harness which feeds the headlight and front running lights. Simultaneously I believe the ignition switch is still powering the brown wire sending power to the tail light so basically the issue is still in the brown wire. Follow voltage from the ignition Key switch to one side of the fuse with the solid brown wire. You should be getting voltage there, the other side of that fuse will send power straight to the tail lamp. There is a bullet connection Under the seat on top of the rear fender with your turn signal connections, green and yellow brake light wire, grounds, etc. Follow that path. You could test both filaments by reversing The bullet connections (with a jumper is necessary due to female and male connectors possibly being different ) just to test the other filament with the brakes. But if you’re not getting power on the brown wire, you just have to figure out if it’s before the fuse, or after the fuse. Could be broken wire somewhere, could be on the ignition switch, could be on the back of the fuse panel. There should be a very linear answer to this problem. Get out the meter.
