Can't get heat through the defroster vent

The fan on my '96 Sebring convertible works at all the speeds (1 thru 4), and I can get both heat, and AC, but I can't select the vent that air comes out of.

No matter what the dial says (high, bi-level, low, defroster and low, defroster), air continues to come out the high vents only. Not a big problem yet, but winter is coming and I think I'll need the defroster.

If any of yall have an idea what's wrong, and think I can fix it myself please let me know.

Thanks.

So I've been rummaging through this site (which is wonderfully extensive, btw), and I'm thinking maybe it has to do with the vacuum lines..?

Where do I find them on my car?

Disconnect the battery for a few minutes. Turn all of the heat/AC controls to the off position. Reconnect the battery and start up. Turn the blower on to whatever speed you need to feel airflow and move the position selector switch to the first position. let it sit there for a few seconds (long enough to determine if airflow is present). Move the switch to the nest position and repeat. My daughters 97 did the same thing and i am told the sometimes the AC controls "forget" where the control doors are supposed to be and the above process will reset it. It worked on her car.