https://www.hayabusa.org/forum/elec...tail-lights-into-integrated-turn-signals.html
This thread should point you in the right direction. I used the info from that thread and ones it points to when modding my tail light.
I cut out a piece of sheet metal and made a divider for my stock tail light so the light is more divided and makes the flashing on just one side possible. It's attached with JB weld.
The side not flashing acts as the brake light, if no flashers are on both lights are the brake light.
I also used some trailer marker lights as amber signals, but you shouldn't need to.
This is the divider
Sorry it's sideways but here is the flasher in action on a sock tail light with the trailer wire harness.
4690b5af.mp4 video by Nytrid - Photobucket
Or you could save yourself the headache and just buy one. Which looking back on it, is maybe what I should have done.