3rd Brake Light

Here you go- CHMSL (3rd brake light) adapter- DIY.

Surely there is a commercial version of this.

Wired like that, if you have a turn signal on and the brakes pressed, the third brake light will flash with the turn signal.

This Curt two wire to 3 wire converter is the easiest way to accomplish a third brake light and if desired, mid way auxiliary turn signals.

https://www.etrailer.com/Accessories-and-Parts/Curt/C56196.html

The wiring diagram posted will only work if it had latching relays because as soon as a turn signal is activated the 3rd light will flash with it. Years ago I made an adapter with Schottky Diodes. It worked but due to the voltage drop across the diodes (~.7VDC) the 3rd light would pulse slightly with the turn signals.

deleted...

Gonzo42 wrote:
I added not only a third brake light, but a modulator. The modulation is adjustable. I have it set so that it flashes 5 times and then comes on solid. I have it mounted to the rack over the spare tire of my toad.

That answers the question, 'is it legal.'
If it's legal in California, it's probably legal in the rest of the Universe. :)

I added not only a third brake light, but a modulator. The modulation is adjustable. I have it set so that it flashes 5 times and then comes on solid. I have it mounted to the rack over the spare tire of my toad.

Should not be hard to do, It should be legal as most pickup trucks have a 3rd brake light.

On GM trucks there is a wire for this purpose in the harness for the 7 pin connector; You would need a wire from the connector to the light itself