The most effective means of removing cockroaches from your car is to use a bug bomb for car. However, there are some alternatives to try out if you want. First off, it's best to first locate where the roaches tend to frequent. This way, you'll know where to lay the traps at. You can then start cleaning your interior, which is as easy as vacuuming every little nook and cranny. Once that's done, you can start deploying the insecticides. These include solutions such as gel bait, borax, or diatomaceous earth. The latter doesn't even require the roach to eat it, as exposure to the solution is enough to kill them. Otherwise, you could also try adding an insect growth regulator alongside those other techniques. That should give your anti-roach solutions more potency.