Get their food supply cleaned out of your car thoroughly. You can buy small ant baits that ants can walk into and remove poison to take back to the nest. Leave them in the car for a while until the ants quit coming back.
they make little ant repelling trays, they are about the size of a silver dollar. i'd get a few of those and place them in the floorboard close to the doors. they dont give off an odor thats strong enough for us to notice so you dont have to worry about harsh smells.