Code:
void create_vehicle()
{
	ENTITY* car = ent_create("crate.mdl", vector(0, 0, 500), NULL);
	NewtonBody *car_body = newton_addentity(car, 80, NEWTON_CONVEXHULL, onforceandtorque);
	NewtonJoint *car_joint = NewtonConstraintCreateVehicle (nworld, vectorf(0, 0, 1), car_body);
}


this doesn't crash for me. my guess is that your n_world pointer is wrong.