<script type="text/javascript">
var arrAutos = new Array();
arrAutos[ 0 ] = ' 1 , 2108 ; 2 , 2109 ; 3 , 2110 ';
arrAutos[ 1 ] = ' 4 , 5  серия; 5 , 6  серия; 6 , 7  серия';

function autoChanged() {
  var autoId = document.asForm.aid.value;
  var arrAutoList;
  var i;

  var modelSelection = document.asForm.mid;
  modelSelection.selectedIndex =  0 ;
  modelSelection.options.length =  1 ;

  if (arrAutos[autoId] == '') {
    document.asForm.mid.disabled = true;
  } else {
    arrAutoList = (arrAutos[autoId]).split(';'); 
    
    for (i =  0 ; i < arrAutoList.length; i++) { 
      modelSelection.options.length = i +  2 ;
      modelSelection.options[i +  1 ].value = (arrAutoList[i]).split(',')[ 0 ];
      modelSelection.options[i +  1 ].text = (arrAutoList[i]).split(',')[ 0 ];
    }

    document.asForm.mid.disabled = false;
  }
}

...

<form name="asForm" id="asForm" method="post">
  <select name="aid" onchange="autoChanged();">
    <option value="0">ВАЗ</option>
    <option value="1">BMW</option>
  </select>

  ...

  <select name="mid" disabled="true">
    <option value="-1">Выбрать</option>
  </select>
</form>

<script type="text/javascript">autoChanged();</script>

modelSelection.options[i +  1 ].text = (arrAutoList[i]).split(',')[ 0 ];

modelSelection.options[i +  1 ].text = (arrAutoList[i]).split(',')[ 1 ];