When we discovered a trail of ants in our house we tried numerouse methods to get rid of them. What quickly solved the problem was Terro Liquid Ant Baits shown below. That will not make a mess in your car as long as you don't flip them on one end and spill the contents. I'm wondering what attracted them in the first place. If they keep showing up is there a nest? Maybe something sweet was dropped and is under the seats.