So what exactly are you trying to accomplish? I only ask because there are a few different ways to do things depending on the overall goal. You can definitely tap the third brake light, but only as a trigger. I would not recommend powering 6 lightheads off of that. The easy answer is a relay. But there are two problems with a mechanical relay. One being that you will hear it click every time you hit the brakes. The second problem, is that they are only good for so long. If this is a car that is going to be in service for 5+ years, and that relay is clicking on and off every time you use your brakes, it might not last. So depending on your setup, the mechanical relay may be best, or you might want to look into something solid-state.