You can also check the actuator. If you pull the glove box, you will see the vacuum "tree" that connects to all of the actuators. I apoloigize but I don't remember which one is for the center vents, hopefully someone can follow up with that, but just hook up a mitivac to the vacuum line for the center actuator and if it holds vacuum, you are good to go. Otherwise... replacing the center actuator requires pulling the dash from what I have read. On the plus side, it could be that the actuator for the defrost vents is broken, in which case its about $30 for a new part and you can service it through the glove compartment.