Change Multiquote Button to Text

[PyroNET]

I'm trying to swap my postbit buttons images for CSS styled buttons and am having troubles with the multiquote button.

Relevant HTML:

<if condition="$show['multiquote_post']">
<li id="mq_$post[postid]">
	<a href="$post[replylink]" rel="nofollow" onclick="return false">$vbphrase[multi_quote_this_message] +</a>
</li>
</if>

Altered portion of vbulletin_multi_quote.js

function change_mq_image(postid, to_selected)
{
	var mq_obj = fetch_object('mq_' + postid);
	if (mq_obj)
	{
		if (to_selected == true)
		{
			mq_obj.className = 'mq_selected';
			mq_obj.innerHTML = mq_obj.innerHTML.replace('+', '-');
		}
		else
		{
			mq_obj.className = '';
			mq_obj.innerHTML = mq_obj.innerHTML.replace('-', '+');
		}
	}
}

I want the script to add the class 'mq_selected' to the element and swap the + for a -, but it currently does neither. Anybody know why?