Combobox carregando Combobox

<script type="text/javascript">

function carrLista(strComboAtual, strComboNovo)
		{			
			xml = '<%=com.sabesp.tiposinformacao.RunTiposInformacao.getXML(request)%>';			
			var el = document.getElementById(strComboAtual);
			var el2 = document.getElementById(strComboNovo);
			var path = "";

						
			removeLista(strComboAtual);
			
			if(strComboNovo == "comboAssuntoNivel1")
			{
				 path = "/taxonomia/categoria[@nivel='1']";

			}else if(strComboNovo == "comboAssuntoNivel2")
			{
			
				if(el.options[el.selectedIndex].text == "")
				{
					return;
				}else{				
					 path = "/taxonomia/categoria[@title='"+ el.options[el.selectedIndex].text +"']";
				}

			}else if(strComboNovo == "comboAssuntoNivel3"){

				
				if(el.options[el.selectedIndex].text == "")
				{
					return;
				}else{		
					path = "/taxonomia/categoria/categoria[@title='"+ el.options[el.selectedIndex].text +"']";
				}
			}

			try //Internet Explorer
			{
				  xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
				  xmlDoc.async="false";
	 			  xmlDoc.loadXML(xml); 			  
	 			  
	 			  var nodes= xmlDoc.selectNodes(path);
			  	  var childNodes;

	 			if(strComboNovo == "comboAssuntoNivel1")
				{
					var childNodes = nodes;	
				}else{
					var childNodes = nodes[0].childNodes;		
				}
			
				

				var y= document.createElement('option'); 
			    y.text= ""; 
		    	el2.add(y);
		    	
				for (i=0;i< childNodes.length;i++)
				{
					y = document.createElement('option'); //cria um novo elemento option
				    y.text= childNodes[i].getAttribute('title'); //seta o texto do elemento option
			    	el2.add(y); // adiciona um novo option antes do indice selecionado para navegadores complacentes
				}	
				sortselect(el2);
				  
			}catch(e)
			{
			  try { //Firefox, Mozilla, Opera, etc.
				  xmlDoc=document.implementation.createDocument("","",null);
				  parser=new DOMParser();				  
				  xmlDoc=parser.parseFromString(xml,"text/xml");				  
				  var nodes  = document.evaluate(path, xmlDoc, null , XPathResult.ANY_TYPE , null);	  
				  var childNodes;
				  var node;
				  if (strComboNovo == "comboAssuntoNivel1") {					  
					var childNodes = nodes;	
					node = childNodes.iterateNext();
					el2.options[el2.options.length] = new Option("", "");					
					while (node) {					  
					  el2.options[el2.options.length] = new Option(node.getAttribute('title'), node.getAttribute('title'));
					  node = childNodes.iterateNext();
			      	}							
				  }else{
					node = nodes.iterateNext();		
					childNodes = node.childNodes;									
					el2.options[el2.options.length] = new Option("", "");					
					for (i=0;i< childNodes.length;i++) {
						 node = childNodes[i];
						 el2.options[el2.options.length] = new Option(node.getAttribute('title'), node.getAttribute('title'));
					}												
				  } 
				sortselect(el2);	
			  }catch(e)
			  {
			  	alert(e.message);
			  }
  			}
		  
		}

</script>

<tr>
					<td>
					<span class="tituloSelectBusca">Selecione um assunto:</span>
				</td>
					
					<td>
						<select id="comboAssuntoNivel1" class="selectBusca" 
						onChange="carrLista('comboAssuntoNivel1','comboAssuntoNivel2')">
						</select>
					</td>
				
				<tr>
					<td>
						<span class="tituloSelectBusca"></span>
					</td>
					
					<td>
						<select id="comboAssuntoNivel2" class="selectBusca"
						onChange="carrLista('comboAssuntoNivel2','comboAssuntoNivel3')">
						</select>
					</td>
				</tr>
				
				<tr>
					<td>
						<span class="tituloSelectBusca"></span>
					</td>
					<td>
						<select class="selectBusca" id="comboAssuntoNivel3">
						</select>
					</td>
				</tr>