Where is the air vent actuator located?

The air conditioning sometimes comes out the defrost vents despite selecting floor or upper vent setting. The problem is intermittent and randomly self corrects. Sometimes it works correctly for weeks and other times it works for a day here or a day there. It doesn't seem to coordinate with temperature or humidity level. I took it to the dealer and of course it was working and they could not diagnose the problem. I can only assume it is a faulty actuator, but cannot locate it to test or replace.