Car bodies are made out of steel. You can prove this by putting a magnet on it. It will stick - which means there is iron (steel is an alloy of iron and carbon).
I don't think car bodies would be made out of aluminium. I doubt its strong enough to widthstand a collision. Magnets won't stick to aluminium. You can test it like that.
There is one car - the DeLorean, who's body is made out of stainless steel. The panels look like a stainless steel fridge!
Some cars also have a carbon-fibre bonnet to reduce weight.