Kill German cockroaches in a car by using a combination of good cleaning habits, roach traps and poison roach bait. Avoid the use of aerosol roach bombs in a vehicle, due to the residual harmful chemicals that become trapped in air vents and on fabrics in the cabin of the vehicle.
The most effective way to eliminate a German cockroach infestation in a vehicle is to cut off the roaches' food supply. German cockroaches eat everything from food crumbs to dirt and paper. Do not eat food in a vehicle that has a cockroach infestation, and be sure to clean the interior of the vehicle thoroughly on a regular basis.
A simple roach bait and poison is made from powdered boric acid, white flour and white sugar. Mix the ingredients together with enough water to form a dough-like consistency, and leave portions of the bait in areas of the vehicle where the roaches are known to frequent. Cocoa powder is an acceptable replacement for white sugar, as both act as bait.
Make a roach killing spray by mixing three parts fabric softener with two parts water, and put the mixture into a spray bottle. Keep the spray bottle in a handy location, so that when a cockroach comes out into the cabin of the vehicle, it can be used to spray and kill the roaches. Aim for the lower midsection and head of the roaches for the best results.