Wondering the same thing since that was not accepted. Are we talking "which car" out of the those sitting in a specific parking lot, or "which car" in the sense of choosing among brands?
Just finished reading "En man som heter Ove" - and this sentence made me think of it and laugh.
I live in the American southwest. We use this question frequently before asking for some kind of favor or errand when the person has access to multiple vehicles -- if you have the ranch truck, could you pick up hay or feed; if you don't have the minivan, you can't pick up the children from school for lack of child safety seats; if you have "my" car, could you check to see if I left my sunglasses; if my son is riding in his friend's car, I may not ask them to stop at the pharmacy to pick up a prescription . . .