Can't override Address Details

Hello,
I am trying to change some words during the checkout process when the customer signs up and enters their address details. I used the developers tool kit and found the file that i was suppose to edit

includes/languages/english/CUSTOM/create_account.php

just to test if this override file works, I was able to change the heading message "If you already have an account with us, please ..".

However, when I change the Address Details title to something like "Billing Address", that doesn't get changed. They are in the same create_account.php file. So whats up?

I uncomment this code, because it was commented out by default:
define('TABLE_HEADING_ADDRESS_DETAILS', 'Billing Address');

Is there another define for that same value in another language file, which is maybe taking precedence?

Thanks DrByte!
It was in english.php file. Next time I'm going to use the very last option to do the search when using developers tool kit. I used the first option and it didn't find that file.