Follow the wire for your rear brake switch up to where it plugs in (UNDER THE TANK). It is a brown connector. The two wires are both black going up to the plug, then on the other side of the plug (on the main wiring harness) the wires are brown and yellow.
If you splice into the black wire that connects to the brown wire, you'll be good to go.
If you splice into the black wire that connects to the brown wire, you'll be good to go.