Hurry! Before beer thirty starts, see if you can get the seat and toolbox cover off. Then you can sit and stare at it for a couple cool ones
Assuming that you don't have some catastrophic harness failure, I imagine it's pretty simple. You're gonna have to start at the fuses and the positive supply end. Since the taillight comes on, it's not the negative side.
The Red/black is the + supply to the ignition switch. The Orange is the supply for the headlight etc. Green goes to the handlebar switches. The Brown is the "taillight only" supply that works in On and Park. If you want to check the switch with a meter or test light, see if you have 12V between the Orange and Ground with the switch turned On.
Like Justin, I don't suspect the switch, but it's a good place to start.
BTW - if you don't have the main supply power, you won't have any control for the starter relay. Don't bother worrying about that. It ain't there