Unfortunately, I had been thinking of my old car, Oswaldo the Death-Trap, who was my first car-love and my home on far too many long road trips. The Death-Trap didn't have cup holders in the back (or, you know, much of anything, either), and I always piled friends in and drove way farther than I should have, so I felt the need!
My new car, as it turns out I had forgotten, has cup holders in the back door arms, and has a fold down central arm rest with cup holders. Oh well. I'll save it for next time.