I had been attempting to implement the Ajax way of implementing dependent drop lower choose boxes inside Joomla. However , should you call an ajaxRequest to operate a php using the query that populates the 2nd drop lower with respect to the consequence of the very first, this has run out of the joomla framework and also you can't use everything from the joomla functions.
So I have to copy in to the new file all of the particulars of my dbase login + to create lower the html+php to populate the choices. I believed whether there's an simpler fix for your problem. Maybe the Joomla framework supports something similar to this.