I have a 2004 XLT with 90,000 miles and the air selector switch for the heat and a/c has stopped working. It will not direct the air flow to any one direction (defroster vents, dash vents, or floor vents), but will flow air to all at the same time. When listening down close to the switch, it sounds as though it's operating when the knob is being turned, but the air flow does not change. Any thoughts? Thank you!

Sounds like it could be the selector.