If you've checked everything here are the directions per KevinX from another site.
1- Remove the seat, Side Covers,
2- Disconnect the Positive Lead on the Battery
3- Remove the Fender-Remove the 4 bolts holding the Fender, place a towel where the saddle was, under the leading edges or lips on the fender.- Spread the fender to clear the fender bolt mounts and tilt it forward from the rear ( No need to remove the Plastic forward mudguard).-Tilt the fender forward but not too far. Use something soft to keep it up to access the taillight (I used an old pillow folded and wrapped in a towel) Be sure that the towel you placed in the front is protecting the leading edges of your fender.
4- Remove the taillight- 1st remove the taillight guard by removing the outer 10 mm bolts, reach up to the top of the guard and work out the two rubber rivets. Unplug the taillight harness. Fold the guard forward and remove the two 10mm bolts on the bottom of the taillight. Reach up again and remove the one 10mm bolt on the side of the taillight with a wrench (Socket wont fit). There is a spring bracket (Two rubber rivets mounted into this) at the top of the light, slightly push it out away from the fender while sliding the taillight down and back. Work it for a minute and the taillight will slide out.
5- Connect 12V Brake power- The brake with (Just above the Oil Filter) is connected with two wires One is Pink/White. This wire sends 12v power to the brake lights regardless if you push the front or rear brakes. It also runs up to the ACM. Splice into this wire and run a wire up through to under the saddle. Make a removable plug connection here then run another wire along the harness to the back where the taillight is. (You can also splice into the ACM at the Pink/white wire. Just be sure your not connecting to red/white or the purple/white)
6- Connect your New Taillight- Cut the harness off of the new Taillight and connect the following wires using a removable connection (You may need to unplug them later) Connect the taillight's RED wire to the wire that was connected to the Pink/White wire you ran from the back. Run a splice on the Purple wire and ad a connection to the taillight. (Feeds the license plate light). This will provide running lights for the taillight. Connect the NEW taillight's BLUE wire to the Purple wire. Remove one of the licence plate bolts an connect a wire to it that will serve as a ground. Connect the taillight's BLACK wire to the new ground that is connected to the license plate bolt. (I recommend all connections be made with good waterproof plugs. I used heat shrink connections and filled them with dialectic grease before plugging them in.)
7- Seal off the old harness- I cut the old plug off of my old taillight since it was blown, taped it up, filled the inside with dielectric grease and plugged it back in.
8-Re assemble the fender- Run Zip ties along the new wire and cable to keep the wire from getting caught in the tire. Put the top 10 mm bolt into the fender and the new taillight, Put the bottom two taillight bolts in and tighten, Put the guard in place, insert the two rubber rivets and tighten the two 10mm bolts into the fender. (Be sure there are no loose wires extruding out past the guard, by using zip ties to secure them) , remove the Pillow and drop the fender into place. Place the fender back onto the mount Realign it with the tire and put the 4 fender bolts back in place. ( Now you can test the connection since the ground was to the license plate bolt nothing will work till its bolted to metal.
9- Test the light- Hook the positive lead to the battery up, turn the power on and test your light. If there is an issue all you have to do is remove the 4 bolts to check it out. If everything is good reassemble your bike.
This taillight cost $155, and $168 after tax. I also connected a Custom Dynamics Night Rider to under the trunk. (The Cause of all my problems.Connecting it to a 12v source then to my taillight burned the 9v LED out) The running light burned out but the flashing Brake light still works so I kept it in place by connecting the red lead to the Pink/White wire and the Black to ground.