CC Clue: Something Missing?

The Pontiac Ventura had two very warm guesses indeed. zykotec was sure it was a ’70 Nova SS, although I specifically showed the part of the taillight that was extended to the center for the Ventura. And csi.ch had an excellent guess of an Opel Kadett B; it does indeed look very much like one. And how I wish I could find one! Eventually…

Anyway, this shot obviously shows something missing; know what it’s from?