SilverML

My 2002 ML320 is has a problem with front defroster operating the duct system to make air blow out at base of windshield. Blower works fine, but when you hit the defrost button on dash, it does not operate the flapper door ducting to make air come out at the base of the windshield. Air blows out the two ducts on each side of the dash but none at windshield base. Turn defroster off and direct air to the windshield, air come out center duct and none at base of the windshield. Anybody with ideas? Has anyone had this problem before?

Thanks,
SilverML