I am new here so sorry if this is in the wrong section/format

I am using Two Spry XML Datasets to populate 2 dropdown boxes with vehicle makes and the connected model names (there are other fields but for the sake of clarity I will only show these 2). The XML is dynamically generated by PHP/MySQL and works great for initially creating records. The problem I have is when I want to edit my record and so pre-select the data in the dropdown select box. The first box is easy and works fine but the second box only updates when the first is updated using the 'onchange' and I cannot get it to update on load. I can get it to update 'onclick' or other events but I need it to update the second box when the page loads. Any ideas? I am sure there is a simple solution staring me in the face, but I just can't see it

My code is listed below. I have cut out all the unnecessary code to make it more clear (hopefully)

<script src="../../SpryAssets/xpath.js" type="text/javascript"></script>
<script src="../../SpryAssets/SpryData.js" type="text/javascript"></script>
<script type="text/javascript">
var dsmakes = new Spry.Data.XMLDataSet("xmlgenmake.php", "vehicle/makes");
var dsmodel = new Spry.Data.XMLDataSet("xmlgenmodel.php?xmlmake={dsmakes::make}", "makes/models/model");

<form action="<?php echo $editFormAction; ?>" method="post" name="form1" id="form1">
<span spry:region="dsmakes" id="makeSelector">
<select spry:repeatchildren="dsmakes" id="makeSelect" name="make" onchange="document.forms[0].modelSelect.disabled = true; dsmakes.setCurrentRowNumber(this.selectedIndex);">
<option spry:if="'{dsmakes::make}' == '<?php echo $row_Recordset1['make']; ?>'" value="{dsmakes::make}" selected="selected">{dsmakes::make}</option>
<option spry:if="'{dsmakes::make}' != '<?php echo $row_Recordset1['make']; ?>'" value="{dsmakes::make}">{dsmakes::make}</option>
</span><span spry:region="dsmodel" id="modelSelector">
<select spry:repeatchildren="dsmodel" id="modelSelect" name="model">
<option spry:if="'{dsmodel::model}' == '<?php echo $row_Recordset1['model']; ?>'" value="{dsmodel::model}" selected="selected">{dsmodel::model}</option>
<option spry:if="'{dsmodel::model}' != '<?php echo $row_Recordset1['model']; ?>'" value="{dsmodel::model}">{dsmodel::model}</option>