New to me 2004.g35 sedan, and the previous owner died, and this black piece was in the back seat. It's approximately as long as the rear windshield. Does anyone know what it is and where it's supposed to go? It has a plastic plug on one end, and little holes spaced along it, with rubber on the other side

Trim piece from the bottom of your front passenger door? (Mirror image of the one below.)
Just a guess.

Thank you so much, that was exactly it, passenger side front bottom inner trim (thing)! But one clip is in the piece, and all the others are gone. So now I have to track down 7 of those clips

Wow, that was a lucky guess. Take a look at one of your other lower-inner-trim-thingys from a different door and maybe even pull one of the center "clips" to see exactly which type it is. Here's a page full of the various clips but I'm guessing it's from the 1st or 2nd row. You may be able to find a generic one that will work from a local auto parts store. Or you can buy a dozen from any Nissan or Infiniti dealer's parts department - or online.