You can try to use "diatomaceous earth" powder. Get food grade variety so its safe if you breathe some of it in.

Sprinkle all over your car surfaces where you've seen the roach walk, as long as the roach walks across it, it will eventually die. It works against most insects, so leave it for the duration of cockroach lifecycle incase got eggs in the car.

Another way is to make cockroach bait using boric acid mixed with something cockroach like to eat. This is highly effective as well, but the bait is toxic, so not an option if you have kids/pets.