//<![CDATA[
function changeMarca(ref)
{
  var selid = ref.selectedIndex;
  var value = ref.options[selid].value;  
  var url = "includes/menuController.php?action=popolaAutoModelli";
  var pars="id_marca=" + value; //+ "&modello=<?=$modello?>"; 
var myAjax = new Ajax.Request(  url,
  {
    method: 'get',
    parameters: pars,
    onComplete: function (data)
   {
      $('selectModello').innerHTML = data.responseText;
    }
  });
}
//]]>