On my 87 300E the center vent (which I recently replaced) was held in place by 4 spring clips near each corner AND an allen screw which is attached to the wheel in the center which opens and closes the volume of air. Don't force anything, but all 5 of the attachments must be undone. Best I recall you can get the allen screw first with a longish allen tool , then the corner clips can be opened with a slender needle nose pliers. good luck,
DanielW