I'm while using get_groups() function to by hand create myself a nav menu. I've got a custom taxonomy I am using known as Category and I am attempting to return the hyperlink for this for my tags within the menu while using get_category_link() function.

foreach ($categories as $category) {
            if ($category->parent == 0) {   //Check to see it is a parent
                $output .= '<li>';
                $output .= '<a href="' . get_category_link($category->cat_ID) . '">' . $category->name . '</a>';  //display parent taxonomy category

             }
      }

However it always returns <a href="">. I'm able to echo the $category->cat_ID effectively so Yes, it is passing the ID in to the function but I'm not sure why it's coming back blank.

Shall We Be Held missing something? Could it be since these are custom taxonomies? They've slugs.

You'll need something similar to this for custom taxonomies:

$tax = 'cars';
  $cats = get_terms( $tax, '' );
  if ($cats) {
    foreach($cats as $cat) {
       $output .= "<li>";
$output .= '<a href="' . esc_attr(get_term_link($cat, $tax)) . '" title="' . sprintf( __( "View all posts in %s" ), $cat->name ) . '" ' . '>' . $cat->name.'</a>';
$output .= "</li>";
    }
  }

Although it is simple to increase the top script to obtain a range of all taxonomies to give in should you wanted.