I used a pair of Ron Francis Brite Lights for my 3rd (& 4th) brake lights. They work only on the brake circuit – brakes, turn signals, and 4 way flashers. With the twin Caddy tail lights this provides 3 lights per side for turn signals, and 6 for braking.
http://www.ronfrancis.com/prodinfo.asp?number=BL%2D69R
