This solution worked for me, and was as cheap as chips:-

Drilled a tiny (~1mm) hole in the bottom of the swan-neck brake light lens, just big enough to take the 2-core cable supplied with
one of these . There's
just enough room to scotch-lock to the two wires inside the light unit (or you could cut and solder), and then snip the cable at a convenient length just outside the lens, replace the lens, and fit your choice of coded bullet connectors, or a small plug and socket from Maplin. The Ebay brake light can be attached to your case (or case cover) with adhesive velcro pads, and removed when not needed. The angle of the brake light array is adjustable. The tiny wire tail, with its connectors, can be tucked out of sight under the swan neck. Voila