My 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" />
    <input type="hidden" name="post_type" value="product" />
    <?php wp_dropdown_categories('taxonomy=product_cat&id=make&child_of=15&show_option_all=Select Make...'); ?>
    <?php wp_dropdown_categories('taxonomy=product_cat&id=model&child_of=21&show_option_all=Select Model...'); ?>
    <input type="submit" id="searchsubmit" value="Search" />
</form>