I drive a 2012 Highlander. The reason you can't put a carseat in the middle is because it's not a real seat...you can fold it up and replace it with a storage thing if you want more storage.
Anyway, I read something somewhere that if you can't do the middle, that the youngest baby should be behind the driver's seat, because you will instictively be more protective of that side. Personally, I like having the carseat on that side because it's easier when running errands.
And to whoever asked, I really LOVE my Highlander. I was driving a CRV before this, and we bought this in January, I've been really happy with it.