You have to start a new listing from scratch, and when they ask you to enter what you're selling, just type in 'car'.

When you get to the next screen, scroll down to the end and click 'continue without selecting a product from the catalogue' (or something similar to that), when you do, a warning will pop up on screen saying that the new listing tool isn't available in the motor category, and that if you continue, you'll have to use the old listing form.

This should only need to be done once, and then all future listings should be with the old form. (Just to note, I have not found any options or ways to opt back in to using the new form, so AFAIK it'll permanently revert you until the new one is fully implemented).