Here is a MOTORCYCLE UNIVERSAL TOP BOX led tail brake light kit. You will have to drill a hole or two to feed the cable down through your top box, depending on the layout of your topbox so. Another way you can go about it is to get a tail light OEM plug that you have on your bike, make up a little extension piece and have the waterproof plug attached to that, It will be more plug and play and your OEM wiring won't be touched or altered.
eBay