ho provato a fare cosi':

<?php
session_start();

if (!isset($_SESSION['make']))
$_SESSION['make_id'] = 0;

if (!isset($_SESSION['model']))
$_SESSION['model'] = 0;

if (!isset($_POST['submit'])) {
$make= $_POST["make_id"];
$model= $_POST["model_id"];
$_SESSION['make']=$make;
$_SESSION['model']=$model;
}
?>
<?php


$query = "SELECT * FROM auto WHERE (make='$make' AND model='$model') LIMIT $x,$y";
$result = mysql_query($query, $db);

?>
<form method="post" action='<?php echo($_SERVER["PHP_SELF"]) ?>'>
<select name="make_id">
<option value="0">...</option>
<option value="14979">ford</option>
</select>
<select name="model_id">
<option value="0">...</option>
<option value="1895">100</option>
</select>
<input type="submit" name="submit" value="Cerca">
<input type="submit" name="action" value="Vedi tutto">
ma non visualizza nulla