Theoretically anything can be a black hole
The most common explanation for how black holes form is through the death of a star, but there are other theories out there. Scientists believe that anything has the potential to turn into a black hole.
The primary element is matter and mass. Thus, a car could turn into a black hole under the right conditions. If a car can be 6 kilometers wide but still keep its total mass, its density would become incredibly high—causing a powerful gravitational force. Of course, this is only speculation since there isn’t any way that we know of to decrease an object’s size down to that volume while also preserving its mass.