The air delivery to the vents are controlled by vacuum pods in behind the dash. The rubber diaphragms inside the pods often crack and break with age causing them to loose vacuum and stop working. More than likely one of the pods are leaking down. I assume you have tried to run the car through all the setting on the climate control?

You should also note that air will only come though the center vents when the wheel is set to the cool side. Even when the A/C is selected and the wheel is set to warm or hot you will still only get air from defrost or corner vents.

Replacement of most of the pods unfortunately requires the removal of the dashboard, which in itself is quite a time consuming job.

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1989 300CE