Need to highlight primary link when secondary link is selected

I got very close with this solution, but it's not working for everything. I have and audio, video, and some other content types. "audio" and "video" were both set up with CCK, and are virtually identical in every aspect. However, for whatever reason, the "audio" tab isn't being highlighted for audio/* and audio/*/* whereas the video tab IS highlighted throughout the video/* and video/*/*. Any ideas? I'm using Drupal 5.3 and modules I have that might be affecting it are: Pathauto and Custom Breadcrumbs.

EDIT: I have figured out a commonality between the content types that are working. They are the only Primary Links that have "No" set in the "Expanded" column of the Menu settings page. All the other menu items don't have anything in that column, and I can't even set them to No unless I put another item underneath them (which still doesn't fix the problem).

The previous comment just clarified things a bit for me...

I don't want to use any secondary links. But I do want my primary links to stay on when, as the previous commentator says, I am in a sub-section.

i.e. I have a section called 'progress' and I would like the 'progress' primary link to remain highlighted for all pages connected to 'progress' i.e. progress/*

This must be easy?!

Can someone clarify?

For those who found the OP's instructions difficult, they do work just fine for D5 once you figure it out - here's a re-cap: find theme.inc and copy the function theme_links() into your template.php, then make the addition commented below with //customization of course changing yourthemename

function yourthemename_links($links, $attributes = array('class' => 'links')) {
  $output = '';

  if (count($links) > 0) {
    $output = '<ul'. drupal_attributes($attributes) .'>';

    $num_links = count($links);
    $i = 1;

    foreach ($links as $key => $link) {
      $class = '';

      // Automatically add a class to each link and also to each LI
      if (isset($link['attributes']) && isset($link['attributes']['class'])) {
        $link['attributes']['class'] .= ' ' . $key;
        $class = $key;
      }
      else {
        $link['attributes']['class'] = $key;
        $class = $key;
      }

//customization - from drupal.org/node/140491

 $explode_array = explode('-',$link['attributes']['class']);
        if(count($explode_array)== 5)
        {
            array_push($explode_array,'active');
            $link['attributes']['class'] = implode('-',$explode_array);
            $pos = strrpos($link['attributes']['class'], "-");
            $link['attributes']['class'] = substr_replace($link['attributes']['class'], ' ', $pos, -6);
        }

// end customization

      // Add first and last classes to the list of links to help out themers.
      $extra_class = '';
      if ($i == 1) {
        $extra_class .= 'first ';
      }
      if ($i == $num_links) {
        $extra_class .= 'last ';
      }
      $output .= '<li class="'. $extra_class . $class .'">';

      // Is the title HTML?
      $html = isset($link['html']) && $link['html'];

      // Initialize fragment and query variables.
      $link['query'] = isset($link['query']) ? $link['query'] : NULL;
      $link['fragment'] = isset($link['fragment']) ? $link['fragment'] : NULL;

      if (isset($link['href'])) {
        $output .= l($link['title'], $link['href'], $link['attributes'], $link['query'], $link['fragment'], FALSE, $html);
      }
      else if ($link['title']) {
        //Some links are actually not links, but we wrap these in <span> for adding title and class attributes
        if (!$html) {
          $link['title'] = check_plain($link['title']);
        }
        $output .= '<span'. drupal_attributes($link['attributes']) .'>'. $link['title'] .'</span>';
      }

      $i++;
      $output .= "</li>\n";
    }

    $output .= '</ul>';
  }

  return $output;
}

Then in your page.tpl.php (this example is from zen starterkit), anywhere you see a call to the theme() function, replace it with something like the code below (sorry for the bizarre php/html code formatting here - can anyone tell me how to make the whole code set display as a single code block?)

if ($primary_links):

// customization from drupal.org/node/140491
// original code: print theme('links', $primary_links); 
 print yourthemename_links($primary_links, array('class' => 'links primary-links'));

// customization from drupal.org/node/140491
// original code: print theme('links', $secondary_links);
print yourthemename_links($secondary_links, array('class' => 'links $secondary_links'));