center brake light

Whether new or used, buy a good one, not cheap junk, and hook it up right so it doesn't flash with the turn signals (or any other time -- 3rd brake light flasher/blinker/pulse devices are available but extremely unwise to install). Be real careful if you buy used; the central brake light has to be mounted so it projects light in a level, straight rearward direction, no matter the angle of the backglass, and you also need to seal it to the backglass so it doesn't cause distracting red glow inside the car (rearview mirror) every time you hit the brakes.

See http://www.allpar.com/reviews/other/LED-third-brake-light.html for an example install on a Valiant.