Don't try to use discount codes that you aren't entitled to, not only is it wrong but you could end up paying a higher rate if caught than if you had simply looked harder for a code you can use.
You can sign up free for Emerald Club on National's website, the code for that is giving the same discount as AAA right now, I'm a member of both and have checked. Alternatively try some of the codes listed on the mousesavers.com website.