Translation:There is a dog in a car behind the office.
Are there different ways of saying the dog is inside the car and on top of the car?
Yep. a dog inside a car: كلب في سيارة - كلب داخل سيارة
a dog on top of a car: كلب فوق سيارة - كلب على سيارة
In regard of the last sentence, the word (فوق) [fawq] might note that the dog is above the car without a contact to the car that is. While the preposition (على) [3alá] is more likely that the dog is directly upon the car. I kept all sentences indefinite to make it general.
your answer would naturally require a comma after "there" : There, behind the office is a dog. Even that actually I don't think it sounds good in English and it would better be: There, behind the office THERE is a dog.