I'm not getting any air flow through the dash vents on my 99 Taurus. When I set the control on A/C or vent, all the air flow goes out the defroster vents. I do get air flow to the floor vents on those settings. I can see the lever that controls the damper move when I change from one setting to another. When I turn it to A/C or vent, it goes to the same position as when I put it on defrost.

Look at this diagram to where the vacuum goes to the defroster actuator. You will probably find a vacuum leak somewhere in the system. The vacuum leak may be under the hood or under the dash as the default position for no vacuum is the defrost mode.
http://usera.imagecave.com/way2old/99TAURUSACPLENUM.gif

Thanks a lot! I'll check that out.

Okay, I'm making progress thanks to way2old's diagram. I located the vacuum motor for the defroster and dash vents. I pulled the yellow vacuum line off the motor. It has vacuum, so everything's okay up to that point. I was able to reach up on top of the motor to the actuator that moves the damper. For some reason, the vacuum isn't able to pull the actuator to move the damper into the position for the dash vents. If I pull it with my fingers toward the vacuum motor, the vacuum starts pulling on it when it gets about halfway, and pulls it on down the rest of the way. Then it stays there until I turn the selector knob to something other than A/C or vent. Does it sound like I need a new vacuum motor, or is there something that might be causing it to hang up? I'm working up through the space where the glove box goes, so I can't see the other side of the motor.

I don't think a new vacuum actuator will help any. Sounds like the door is binding for some reason. You might try a little lubricant where the shaft comes through the plenum to see if that helps. I have seen a few of the doors break on these. Not many, but a few.