How to use it with Ubercart Products?

hi,
I'm not sure about your question but I think you can use the method in the module read me file.

first configure it in Administer › Site configuration >Prev/Next.
for product you can change the code in readme file like this.you can add a code to template.php file of your theme:

function pn_node($node, $mode = 'n') {
  if (!function_exists('prev_next_nid')) {
    return NULL;
  }

  switch($mode) {
    case 'p':
      $n_nid = prev_next_nid($node->nid, 'prev');
      $link_text = 'previous';
      break;

    case 'n':
      $n_nid = prev_next_nid($node->nid, 'next');
      $link_text = 'next';
      break;

    default:
      return NULL;
  }

  if ($n_nid) {
     $n_node = node_load($n_nid);
		switch($n_node->type) {
      // For image nodes only
      case 'product':
	   $html = l($link_text, "node/$n_nid", array('html' => TRUE));
       return $html;
      default:
        // Add other node types here if you want.
    }
  }
}

then copy node.tpl.php and rename it to node-product.tpl.php and add html code like this which is shown in readme file.

  <ul id="node-navigation">
    <li class="next"><?php print pn_node($node, 'n'); ?></li>
    <li class="prev"><?php print pn_node($node, 'p'); ?></li>
  </ul>

hope this helps,

I think you can start by changing the pn_node method. I changed it a little may be that helps. you may need to change the path.

function pn_node($node, $mode = 'n') {
  if (!function_exists('prev_next_nid')) {
    return NULL;
  }

  switch($mode) {
    case 'p':
      $n_nid = prev_next_nid($node->nid, 'prev');
       break;

    case 'n':
      $n_nid = prev_next_nid($node->nid, 'next');
       break;

    default:
      return NULL;
  }

  if ($n_nid) {
     $n_node = node_load($n_nid);
	  $options = array(
      'attributes' => array('class' => 'thumbnail'),
      'html'  => TRUE,
    );
		switch($n_node->type) {
      // For image nodes only
      case 'product':
	   $html  = '<img src="'.base_path().'sites/default/files/imagecache/product/' . $n_node->field_image_cache[0]['filename'];
       $html .= '" class="image image-thumbnail" />';
        $img_html = l($html, "node/$n_nid", $options);
	   $text_html .= l($n_node->title, "node/$n_nid", array('html' => TRUE));
       return $img_html . $text_html;
      default:
        // Add other node types here if you want.
    }
  }
}

I've modified the code of module in order to include taxonomy. you can make these changes to see if it solves the problem. I suggest you to make a backup first.this may not solve all of issues. please let me know if it works.
in prev_next.module file replace this function.

function _prev_next_add($nid) {
  $node_type = db_result(db_query("SELECT type FROM {node} WHERE nid = %d LIMIT 1", $nid));
  $search_criteria = variable_get(PREV_NEXT_NODE_TYPE . $node_type . '_indexing_criteria', PREV_NEXT_INDEXING_CRITERIA_DEFAULT);

  $criteria_value = db_result(db_query("SELECT %s FROM {node} WHERE nid = %d LIMIT 1", $search_criteria, $nid));

  $cond = _prev_next_node_types_sql($node_type);
   $node = node_load($nid, NULL, TRUE);
	$mytid= end($node->taxonomy)->tid;
  $next_nid = db_result(db_query("SELECT n.nid FROM {node} n INNER JOIN {term_node} r ON n.nid = r.nid WHERE %s > '%s' AND r.tid = ".$mytid." AND status = 1 $cond ORDER BY %s ASC LIMIT 1",
    $search_criteria, $criteria_value, $search_criteria));


  $prev_nid = db_result(db_query("SELECT n.nid FROM {node} n INNER JOIN {term_node} r ON n.nid = r.nid WHERE %s < '%s' AND r.tid = ".$mytid." AND status = 1 $cond ORDER BY %s DESC LIMIT 1",
    $search_criteria, $criteria_value, $search_criteria));

  // Update the node-level data
  $exists = db_result(db_query('SELECT COUNT(*) FROM {prev_next_node} WHERE nid = %d', $nid));
  if ($exists) {
    db_query('UPDATE {prev_next_node} SET prev_nid = %d, next_nid = %d, changed = %d WHERE nid = %d',
      $prev_nid, $next_nid, time(), $nid);
  }
  else {
    db_query('INSERT INTO {prev_next_node} (prev_nid, next_nid, changed, nid) VALUES (%d, %d, %d, %d)',
      $prev_nid, $next_nid, time(), $nid);
  }

  // Update the other nodes pointing to this node
  foreach (node_get_types() as $type => $name) {
    if (variable_get(PREV_NEXT_NODE_TYPE . $type, 0)) {
      $search_criteria = variable_get(PREV_NEXT_NODE_TYPE . $type . '_indexing_criteria', PREV_NEXT_INDEXING_CRITERIA_DEFAULT);
      $criteria_value = db_result(db_query("SELECT %s FROM {node} WHERE nid = %d LIMIT 1", $search_criteria, $nid));
	  
      $cond = _prev_next_node_types_sql($node_type);
	$node = node_load($nid, NULL, TRUE);
	$mytid= end($node->taxonomy)->tid;
      $next_nid = db_result(db_query("SELECT n.nid FROM {node} n INNER JOIN {term_node} r ON n.nid = r.nid WHERE %s < '%s' AND r.tid = ".$mytid." AND status = 1 $cond ORDER BY %s DESC LIMIT 1",
        $search_criteria, $criteria_value, $search_criteria));
      if ($next_nid) {
        db_query("UPDATE {prev_next_node} SET next_nid = %d WHERE nid = %d", $nid, $next_nid);
      }
      $prev_nid = db_result(db_query("SELECT n.nid FROM {node} n INNER JOIN {term_node} r ON n.nid = r.nid WHERE %s > '%s' AND r.tid = ".$mytid." AND status = 1 $cond ORDER BY %s ASC LIMIT 1",
        $search_criteria, $criteria_value, $search_criteria));
      if ($prev_nid) {
        db_query("UPDATE {prev_next_node} SET prev_nid = %d WHERE nid = %d", $nid, $prev_nid);
      }
    }
  }

}

may be you need to re index the site in Administer › Site configuration >Prev/Next

Is this still an issue for you? If so, feel free to reopen.