it's cheap and effective:

I nearly got rear ended by a builder in his van at a give way a few months ago and thought that a high level brake light should be a useful attention grabber for even the dopiest of drivers...

Bought a 3euro LED brake light strip from China, sowed it to the back of my jacket with clear fishing line which I then used hot glue to seal on the inside of the jacket.
Ran the LED cable down the inside of the jacket lining and bought it out in the inner pocket, attaching a quick release female connector.

Spliced a cable into the brake wiring on the bike fitting the male on the end - I used a flexible cable so that when I got off the bike I still had a few feet of travel available.

The male connector I run under the jacket to the female inside connector - out of the weather etc..

Works well - anything helps...