The taillight wiring is so complicated because it supports the Park function, where the switch connects the red 12+ wire from the main fuse directly to the brown TL2 taillight wire, and allows the key to be removed. I have never used this function, and the only reason I can think of why it was included on every Honda had something to do with other markets? It wasn't a US requirement, as far as I know.
When the key is in the On position, 12v goes into the switch on the red wire, and comes out on the black wire, which provides power, in turn, to the brown/white TL1 wire, which then provides power back to the switch to the brown TL2 wire through the switch contacts. If there isn't 12V on the brown/white wire with the switch on, there is a problem besides the switch. Where in the harness the brown/white wire connects to the 12v supply from the black wire varies; if your bike has a headlight on/off switch, that switch connects the black wire to the brown/white wire, which goes on to the headlight fuse, tach and speedo lights, and taillight. Simple, right??
