You're gonna need to get out a voltage-tester or multimeter and figure out what wires on the bike do what. If memory serves me, there really ought to be only 3 wires involved. One is ground, the others are probably tail +, and brake +
If you have a multimeter, connect the black lead to ground and the red lead to the one you suspect is your brake light wire, then turn on the key and grab the front brake lever or step on the rear brake lever. Should go from 0v to ~12.5V.