What KIND of ants? Different species need different baits. Your biggest problem will be keeping the car out of heat & direct sun so the bait doesn't overheat.
Gonna take a guess that you have little brown ants which eat sweets. The right bait (the kind they take back to the queen) oughta work well in a couple weeks. If it's a soda spill or the like, you need to clean that up or you'll just keep getting them back again.
I like permethryn. It is the active ingredient in bug repellent clothing. You can buy stuff that you soak clothing in - sawyer is one brand. It is also the main ingredient in a lot of the nonaerosol bug sprays for your house. You can also get it cheap in concentrated form at farm stores. I haven't found the stuff I have used to stain. Has a very slight odor some describe as being like patchouli. It is dangerous to cats if they ingest it when wet, but no problem when it is dry.
We had it happen in our vans after camping for several months in them. Terro liquid ant bait is borax based and did the job. You can mix it up yourself from borax and sugar but for a couple of bucks....