Wordpress - Multiple Dropdowns that Require each other
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Can anyone help really quick with a simple Wordpress question?

How do you have multiple dropdowns that require each other? 

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Ex. If you pick the year 2011 category but pick a category with no posts in it, it should show no posts - instead it shows the 2011 posts.

    <form action="<?php bloginfo('url'); ?>" method="get">
         <?php wp_dropdown_categories( ('show_option_none=All Years&child_of=19') ); ?> 
         <?php wp_dropdown_categories( ('show_option_none=All Makes&child_of=16') ); ?>
         <?php wp_dropdown_categories( ('show_option_none=All Models&child_of=17') ); ?>
         <?php wp_dropdown_categories( ('show_option_none=All Trims&child_of=18') ); ?> 
         <?php wp_dropdown_categories( ('show_option_none=All Body Styles&child_of=15') ); ?> 
         <?php wp_dropdown_categories( ('show_option_none=Any Price&child_of=20') ); ?>              
      <input type="submit" name="submit" value="view" />
    </form>



Any help is greatly appreciated! 