First you need to do a special shape by grouping all car objects in one.(eg: car)
Then you select this object "car", select polys by texture, detach them and create a new object named eg:texture 1
Redo it for each texture of the car
So at the end you have a special shape with somes objects named texture1, texture2, etc....
select only the object texture1 and render as you did... but this time only texture 1 will be rendered.
et voilĂ ......
if you want to do rendered shadows, replace the original textures of the car by a full grey texture (128,128,128)
before to render it