I think it should be
هناک خلف البیت سیاره
Because it is indefinite ("a car" not "the specific car") I think they prefer not to use هناك in this kind of sentence. (When the prepositional phrase comes first, at least.)
You can write هناك if you want but it is not obligatory...
i appreciate your contributions to this board. you've been super helpful
هناك سيارة خلف البيت
please stop with mixing letter types! I am about to quit my beloved course because of this!