3rd brake light.
 

Hello all, I just picked up a third brake light for my 85 s3 where can I splice into my wiring, is there a thread already posted.

I would run the power wire back from one of the existing brake lights up to your third light. You can probably get ground from the existing wire up there running from the dome lights and rear window heater. Ground wires are solid brown. I saw a site that suggested running it all the way down to the battery ground, or also to the brake light, which I personally don't consider necessary.

One thing to keep in mind is that the bulb monitoring relay is designed to sense a balanced L/R current draw on the 85 cars (928 641 603 04), so adding a second bulb on one brake light circuit will likely cause the brake light bulb warning to always trigger.

If you look at the wiring on the 86 car which came with the 3rd brake light from the factory, it appears that it is tied into the system in parallel with the Right stop light and the bulb monitoring relay changed to part number 928 641 603 05. presumably the new monitoring relay is designed to have twice the current draw on the right circuit and not trigger the warning,

If it were me I would tie into the right stop light as Porsche did on the 86 cars, and consider looking for a 928 641 603 05 monitoring relay if the stop lamp warning is continuously lit.