Yup. A good one, mounted as high as possible (top of the backglass is better than bottom of the backglass, but bottom of the backglass is better than nothing). See here and here for new ones. If you adapt one from a wrecking yard, pay careful attention to the angle -- the lamp, when installed, needs to be looking straight rearward, not up or down.
And hook it up correctly, so it goes solid-red when you step on the brake. There are all kinds of gadgets that blink/flash/"pulse" the central brake light, with baseless claims that they improve safety. In fact they do the opposite, so steer clear of 'em.
