Had the exact same problem on my '99. Here's what I found the cause to be and how I fixed it:
The defrost vent door is connected to the vaccum actuator with a little platsic thingee that has a star shaped shaft that had slipped out of the vent door shaft. The only thing that holds this part in place is the arm coming from the vaccum actuator. The fix was to bend the arm (gently) toward the air box so
that it held the part that had slipped out into the vaccum door shaft. You have to pull the dash all apart to get to this and even then, it's a PITA to get to the vaccum actuator shaft you need to bend. Good luck...
