The filters are behind the glovebox. It will have to be removed to get to them.
They are supposed to be replaced quite frequently (at least every 30k miles). If they get plugged the airflow through all ducts will be reduced... does your A/C work well?
To verify the function of the various duct positions check this article and do the duct control test: http://www.babcox.com/editorial/ic/ic40142.htm