Here's how to make one, these "Stealth 3rd Brake Light" directions are for BMW E9s and 2002s, so your measurements for extension wires may differ.
Here's how to make one and install it.
The 3rd brake light is an LED strip 1 foot long Alpena 180°red with 60 inches of additional red & black wire soldered with heat shrink insulation at the joints. Then red & black alligator clips soldered to the wire ends. Then put little pieced of electrical tape around the wires to hold them together. It attaches to the upper center of the rear window seal with the wires run under the lip of the seal to a hole under the parcel shelf. Then under the white fender welting, under the gas tank trunk board to the brake light tab and ground on the passenger side tail light.
Need a soldering iron, flux, solder, Alpena LED, extension wires, heat shrink tubing, electrical tape and alligator clips or tap into brake light circuit and ground.
Alpena lights are available at Advance Auto Parts or on ebay. 180s, have white logos and cut lines which I color out with Industrial Grade Sharpie
I used to buy the extension wire, heat shrink tubing and alligator clips at Radio Shack.
If you have any questions I'm happy to answer.