Sorry it took so long to get this done, but here it is!
The Bestem top case is an amazing value at $80 shipped. Once you get it mounted (most people, myself included, have to drill extra holes in the mounting plate - there are other guides for that), you still need to get the extra brake light wired up. This is what I will cover here.
Notes: The ads for this box claim the light is LED. It is not, it is a row of several (4 or 5) incandescent bulbs. I may in the future write a guide to converting to a LED system. Also, the wire and connectors that this case comes with are pretty flimsy, I replace their connectors with something more substantial. Also, these pics were taken by my cell phone, and you know how that goes. I'm missing a few pics because they didn't come out, but i think you'll get the idea.
Required parts: I bought all supplies at Radio Shack. A real electronics store would have been better, but less repeatable for folks who aren't familiar with electronics.
With that in mind, here is what I used:
. Spool of 22 gauge speaker wire
. 2-position interlocking quick disconnect connector (part # 274-222)
. Tap-In Squeeze Connectors (# 64-3053)
. Black Electrical Tape
. Wire strippers
. Soldering Gun (equivalent to # 64-2187, I got mine at Autozone)
. Remove seat. Unmount top case, remove grab bar/rear rack
. Remove allen bolts holding rear fender/lights. One on each side, like so:
. Remove rear fender/light housing. The trick is to pull the sides away where the allen bolts go, then it slides right off.
. Cut a length of about 18 inches of the speaker wire. Split about 3" of wire on each end and strip around 1/2 inch on each end. The Quick Connect package has 2 plastic housings and 4 metal connectors. Separate the male and female connectors, we will use the females first. These crimp onto your wires, just slide the wires into the back and squeeze it with the pliers. For good measure, I put a bit of solder on it. You may may need to use the pliers to make the connector more round, as it has to be pushed up into the plastic housing. Once you are done, it should look like this:
Run the other end of the wire into the wire hole on the top case. Cut the bullet connectors off of the end of the existing wire and strip them down. use your soldering gun to solder red to blue and black to yellow. Then, wrap the connections up with electrical tape. Not pictured, i also tied a knot in the wire to keep it from being pulled through the hole.
. Locate the wires running to the tail light. The ones we're interested in are the white with black stripe (positive) and the black with white stripe (negative).
. Cut another length of about 18 inches of the speaker wire. Split about 3" of wire on each end and strip around 1/2 inch on each end. On one end, we need to connect the Quick Connect male pins, similar to the female pins, and push them through the other direction on the plastic housing. Make note to match the positions of the colors so when you plug it together, the red wire connects with the red wire on the other connector.
. Open your wire taps package. The wire taps have 2 compartments, one goes all the way through and the other goes 3/4 and stops. To connect the positive lead, slip the tap connector onto the white with black stripe wire, and stick the end of the red wire into the part that only goes 3/4 through. Once these are in, you put pliers around it so that you're pushing the metal tab straight in. Give it a good squeeze and it should slide down and tap the wires. Fold the plastic retention clip around it and you should be good to go.
Repeat this step with the black with white stripe wire on the bike and the black side of the speaker wire. it should look like this:
For good measure (and some waterproofing), wrap those taps in electrical tape. Stretch the tape slightly while you're applying it for best results.
Not pictured, I wrapped some electrical tape around the back of each of the plastic housing to keep water and dirt from getting in.
At this point, you should be able to test your light. Plug the Quick Connectors together, turn on the ignition, and pull the front brake. Your case light should come on along with your regular brake light. If not, a multimeter would be very handy to determine where a fault lies. Check for 12v DC on the Quick Connector pins, if this is good check for it where you soldered the speaker wire to the top case lighting wires, and redo any connection that isn't flowing electricity.
Assembly is much like disassembly. Disconnect the Quick Connector and set the case aside. Putting the fender/tail light housing is similar to removing it, pull the sides out to slip over the raised areas where the allen bolts go, while paying attention to your alignment with the side bodywork. It pretty much will snap in to place when you have it right. Put the allen bolts back in, then you can mount your grab bar/luggage rack again. Mount the top case to the rack and run the lighting wire under the grab rail, plug it in and leave any extra wire under the seat. Put the seat on and test your light again. If everything went properly, you're done!