Yup, almost made that mistake. I mean, no casual observer would probably actually see it, let alone know it wasn't supposed to be there, however, I would....Anyhow, I just took some old sprue and shaved it down to the correct size and filled the holes for it, and then sanded it flush with the floor.
Also, I've learned to never trust the painting instructions for the instrument panels and instead go off pictures I find on the internet. However, I can't for the life of me find a decent picture of the back seat instrument panel for a B/N version.