How to access a field in an Iframe or a HTML webresource from the CRM form

Here I am trying to access a field on the webresource that I have placed on the mscrm 2011 form.
First we get to the web resource and then get to the field within it

 

 

window.document.getElementById("WebResource_TimePicker").contentWindow.document.getElementById("wrapper").style.visibility = 'visible';