Relative Path From An Included PHP File Within Theme Directory

[noxbane]

So I'm using a custom Genesis child theme. I've created my own PHP file called showcase.php and placed it in my custom child theme directory.

This file is located at: /public_html/wp-content/themes/mycustomtheme/showcase.php

This file is included via a hook:

PHP Code:

add_action('genesis_after_header', 'child_make_showcase');
function child_make_showcase() 
{
    require_once(STYLESHEETPATH.'/showcase.php');    
} 


Within showcase.php, I am changing the background of an element using Javascript when the user clicks on a certain area of the page:

document.getElementById("ED-quicksearch").style.backgroundImage = 'url("/wp-content/themes/mycustomtheme/images/quicksearch-bg-tucson.png")';

That's just part of the Javascript of course. I swear the above code worked before but now it doesnt. I can't seem to figure out what path to give it. If I give it an absolute URL it works, so the image does exist in the proper place and the element ID is correct, I just can't figure out what relative path to use. And it boggles my mind that I am certain it USED to work with that relative path above. Neither Firefox Error Console or Firebug show javascript errors.