I have never built a turn signal unit, but I have built my own brake light. I used the cheapo turn signals, but they broke and now I just use hand signals. I have a lever with a built-in brake light switch now, but I'll tell you what I used for a light and how I wired it originally. I used an old Yamaha turn signal. The body of the turn signal is metal, so it grounds to the frame automatically. I attached it to my luggage rack by drilling a hole through and bolting it on. I wired the positive wire from the light to my battery. I then made the negative wire longer so I could run it up to my brake lever on my handlebars. I made a tab that I could wedge in between my handlebar and my brake lever, and rigged the ground wire to the lever so that when I squeezed the lever, the wire would touch the tab, thereby completing the circuit and activating my brake light. Releasing the lever broke the circuit, turning the light off again. It's crude compared to a proper brake light switch, but it works.