This is the beginning of the file in question.
PHP Code:
<?php
/**
* Side Box Template
*
* @package templateSystem
* @copyright Copyright 2003-2006 Zen Cart Development Team
* @copyright Portions Copyright 2003 osCommerce
* @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
* @version $Id: tpl_categories.php 4162 2006-08-17 03:55:02Z ajeh $
*/
$content = "";
$content .= '<div id="' . str_replace('_', '-', $box_id . 'Content') . '" class="sideBoxContent">' . "\n";
[COLOR="Red"] for ($i=0;$i<sizeof($box_categories_array);$i++) {[/COLOR]
switch(true) {
The highlighted portion is the line of code you need to find.
If your category sidebox is functioning correctly that line of code should be there. If not I'd suggest you upload a clean copy of this file.