Theme Code

<form method="get" id="searchform" action="<?php echo home_url(); ?>">
    <input type="text" onclick="this.value='';" onfocus="this.select()" onblur="this.value=!this.value?'Search...':this.value;" value="Search..." name="s" id="s" />
    <?php
        wp_dropdown_categories(
            array(
                'child_of' => 15,
                'id' => 'make',
                'name' => 'product_cat',
                'show_option_all' => 'Select Make...',
                'taxonomy' => 'product_cat',
                'walker' => new dropdown
            ))
    ?>
    <?php
        wp_dropdown_categories(
            array(
                'child_of' => 21,
                'id' => 'model',
                'name' => 'product_cat',
                'show_option_all' => 'Select Model...',
                'taxonomy' => 'product_cat',
                'walker' => new dropdown
            ))
    ?>
    <input type="submit" id="searchsubmit" value="Search" />
</form>