Even if there's no food inside the car, a cockroach can survive on anything it can gnaw on. carpets, wire insulation, die-electric grease, etc. You name it, if its soft enough to chew upon, they can eat it. There are roach traps being sold at groceries and mall hardware stores, IMHO those are effective too.
I once bought a car that smelled of rat piss inside, no matter how I cleaned it .. it still smelled the same! So I took some sticky fly paper and dabbed peanut butter in the middle. Early the next morning, the thing caught not one, but three (3) mice!
Got rid of that car before I died of leptospirosis!