aqui va el codigo la idea es que al seleccionar la clave se llenen los <input segun lo que yo elija en el select
<HTML>
<HEAD>
<TITLE>Actualizar1.php</TITLE>
</HEAD>
<BODY>
<div align="center">
<h1>Actualizar Los Datos del Vehículo</h1>
<br>
<?
$myconn = mysql_pconnect("localhost","","");
$bd = mysql_select_db("AUTOMOTORA",$myconn);
$sql = "SELECT patente,marca,fecha,stock FROM AUTOS ORDER BY patente";
echo '<FORM METHOD="GET" ACTION="actualizar2.php">Seleccion patente<br>';
$result = mysql_query($sql);
echo '<select name="mpatente">';
//Genera menu desplegable
while ($row=mysql_fetch_array($result))
{echo '<option>'.$row["patente"];}
?>
</select>
<br>
<br>
Marca<br>
<INPUT TYPE="TEXT" NAME="marca" type="text" VALUE="<? echo $marca ?>" size="10" maxlength="10"><br>
Fecha<br>
<select name="dia">
<Option>01</option>
<Option>02</option>
<Option>03</option>
<Option>04</option>
<Option>05</option>
<Option>06</option>
<Option>07</option>
<Option>08</option>
<Option>09</option>
<Option>10</option>
<Option>11</option>
<Option>12</option>
<Option>13</option>
<Option>14</option>
<Option>15</option>
<Option>16</option>
<Option>17</option>
<Option>18</option>
<Option>19</option>
<Option>20</option>
<Option>21</option>
<Option>22</option>
<Option>23</option>
<Option>24</option>
<Option>25</option>
<Option>26</option>
<Option>27</option>
<Option>28</option>
<Option>29</option>
<Option>30</option>
<Option>31</option>
</select>
<select name="mes">
<Option>01</option>
<Option>02</option>
<Option>03</option>
<Option>04</option>
<Option>05</option>
<Option>06</option>
<Option>07</option>
<Option>08</option>
<Option>09</option>
<Option>10</option>
<Option>11</option>
<Option>12</option>
</select>
<?
$inicio=1800;
$final=2010;
echo '<select name="ano">';
for ($i=$inicio; $inicio <= $final; $inicio++) {
echo '<option>'.$inicio;
}
?>
</select>
<br><br>
Stock<br>
<INPUT TYPE="TEXT" NAME="stock" type="text" VALUE="<? echo $marca ?>" size="10" maxlength="10"><br>
<br><br><br><br>
<INPUT TYPE="SUBMIT" value="Actualizar">
</FORM>
</div>
</BODY>
</HTML>