使用ajax调用时，不会填充MySQL查询数组

I'm using ajax to return the results of a sql query on a WordPress site. Basically, there are two queries and I am displaying the results of the second query. The first query gets a list of posts that contain a certain custom field. The second query looks through those posts and lists out the posts that have another custom field associated with them. Right now, if I don't use ajax and just add this to my page template, everything works.

<?php
    $post_ids = $wpdb->get_col( "SELECT DISTINCT post_id FROM $wpdb->postmeta WHERE meta_key = 'Make' AND meta_value='Chevrolet'" );
    // now get the food names
    $stocktypes = $wpdb->get_col( "SELECT DISTINCT meta_value FROM $wpdb->postmeta WHERE meta_key = 'Model' AND post_id IN (". implode(',', array_map('absint', $post_ids) ) .")" );


    if ($stocktypes) {
        foreach ($stocktypes as $stocktype) {
            echo "<option value=\"" . $stocktype . "\">" . $stocktype . "</option>";
        }
    }
?>

The problem comes when I add that to a separate file and call it with ajax. Here is the contents of my ajax file:

<?php

    $models = $_GET['Make'];

    $post_ids = $wpdb->get_col( "SELECT DISTINCT post_id FROM $wpdb->postmeta WHERE meta_key = 'Make' AND meta_value= %s", $models );

    $stocktypes = $wpdb->get_col( "SELECT DISTINCT meta_value FROM $wpdb->postmeta WHERE meta_key = 'Model' AND post_id IN (". implode(',', array_map('absint', $post_ids) ) .")" );


    if ($stocktypes) {
        foreach ($stocktypes as $stocktype) {
            echo "<option value=\"" . $stocktype . "\">" . $stocktype . "</option>";
        }
    }
?>

Here is my ajax call:

 <script>
    $(document).ready(function(){
        $("#make").change(function() {
            $.ajax({ 
                url: "<?php bloginfo('template_url'); ?>/getModel.php", 
                data: { Make: $(this).val() },
                dataType: "html",
                type: "GET", 
                success: function(data){ 
                    $("#model").html(data); 
                } 

            }); 
        });
    });
</script>

The ajax call is working fine because I can echo the results of $models = $_GET['Make'];

By using print_r(array_values($post_ids));, I found that the $post_ids array of the first query isn't being populated in my ajax file. The only thing I could think of is that wp-load.php (which allows for the use of WordPress functions) wasn't being loaded but that's not the problem.