WP Form with Conditional Fields

Hey everybody,
we a trying to make a very simple webpage and the only thing we want on our site is a form.
this form should contain some buttons and some dropdown-menus. normaly this should be no problem for me. but my brother asked me to make the 2 of the dropdown-menus depending in each other. for example the form on mobile.de, you klick on the marke dropdown and select a car, then the second dropdown gets active and shows you every model from the car you selected. we want exactly the same things and yes it should contain cars and their models.
i try to use contact form 7 with Conditional Fields but it seem to work only for 1 field in the dropdowns and not for more. do someone know any plugin that can do that or has any advice? is it so hard to do?
thanks for help
Cheers
adam