Custom third brake lights are available from aftermarket automotive parts stores or on the Web at CARiD or AutoAnything, and the units are relatively easy to install. If the vehicle owner is unable to perform the installation himself, a mechanic or auto electronics specialist can complete the task.
The least expensive and most common location to install a third brake light is inside the vehicle, on the rear dash or attached to the roof above it, as close as possible to the rear window. Another popular location for a third brake light is on a custom rear spoiler. These usually must be painted to match the vehicle and thus professional installation is recommended.
Third brake lights designed for the vehicle interiors usually include mounting brackets, hardware and installation instructions. Mounting is straightforward though it involves driving screws or other hardware into the vehicle's interior. The light has three or four wires that should all run to the trunk. A good place to run the wires is beneath upholstery and behind the rear seats so that the wires do not show and cannot be bumped by passengers.
Once the wires are run to the trunk, they can be spliced into their corresponding wiring found on the rear of one of the car's factory brake lights. There is a power wire, a ground wire (this can be attached to any metal part of the vehicle's chassis) and a wire for the brake light switch. Consult your vehicle's maintenance guide and the light's instructions to ensure the wiring configuration is correct.