Translation:There is a dog in a car behind the office.
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.