If you are asking about the brake light circuit only, it is a grn/purple wire on the bike harness side going into pin 1 on the plug. Pin 1 then connects to blue on the tail light side of the plug.
If you look at plug on the bike harness side:
Pin 1 (grn/purple) is brake light.
Pin 2 (blk) is neutral.
Pin 3 (yel) is taillight.
Or you could just exercise some discretion and use a test light.
This info is for an 05-07 sprint. I wish people would add their bike to their profile or identify it in their post.