The problem is you have too many solids and surfaces.  You want one solid to represent the car before you add the air solid around the car. Do you have a CAD system that can unite all the solids and fill in any gaps?  You could use SpaceClaim or DM to do that too.

Are you on the Student license? It has a limit of 300 faces, which this geometry exceeds. If you are on a full license, there is not a problem with the number of faces, but you can get a smaller model to solve if you get rid of all the grill details on the back, if your interest is in the air flow around the body.