Good Afternoon, I have a 1989 Z-71 whose defroster does not work. I get plenty of heat and full force fan from the center vents but not the top defroster vents. I changed the switch on the front of the "box" that controls an internal flap. I have the dash semi dismantaled but I do not see any other switches that may be the culprit. Can someone offer what else to check or what the real culprit may be? Thank you for the help.