Are you sure you need it? You can just use Aref=1m². Then your Cx results will already be Cx*A, which is what you actually need to calculate the forces.
Another option is creating an image of the front view, but keep everything black with a white background and then with some Python or other simple language you can count the pixels and with that determine the area.
Or if you have stl files of your car with Paraview you can do it as well:
This method however does not work very well with large and complex stl files but a F1 in school car should be fine.