CRM dynamics 2013 form load event does no set IFRAME src url

The following code does set the IFRAM url src in crm 2011 but in 20123 when we upgraded it does not work.

in the IFRAME url it's self it does work but I cannot change it from the form load event java script

function GetCustomerNumber(){

var el = document.getElementById("IFRAME_OrdersList");
    var el2 =Xrm.Page.getAttribute("new_customernumber").getValue() ;
   var url= "http://dev-appsvr/CrmCSC/OrderList.aspx?cn=" + el2;
//el.src=url;
if(Xrm.Page.ui.controls.get("IFRAME_OrdersList")!=null)

{
el.src=url;

}
else
{
alert('frame not loaded');
//window.setInterval("GetCustomerNumber()",1000);}
}