Huh, you all may have saved me some trouble. I looked all over to see if maybe we just had one light, but every photo I saw from the back showed two lights.
Had someone point out that my brand new z only had one back up light come on when I was reversing. Just took delivery two weeks ago, so likely an assembly mistake, but a reminder to get someone to look at your lights when you back up to make sure they are installed correctly.