URL rewrite help required

I am using a classified theme that passes some variable in the url.
I want to achieve
http://www.abc.com/ad-category/cars/Paris
from
http://www.abc.com/ad-category/cars/?cp_state%5B%5D=Paris

I tried putting this before wordpress rules in htaccess file

RewriteEngine On
RewriteRule ^/ad-category/cars/([a-zA-Z0-9_-]+)$ /ad-category/cars/?cp_state[]=$1 [NC,L,QSA]

but nothing happens. No error either.

[ No bumping please. ]