If I remember there is a test procedure for the center vents that involves shutting the car off, setting the particular selections on the pushbutton unit (might be two buttons at the same time) and then turning on the key and the center vents will blow immediately if the system is working correctly.

Now as far as I can tell they are somehow triggered by outside or ambient temp because in the winter, with the car nice and toasty inside and I flip over to the coldest setting and hit the compressor it will still not blow through the center vents.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique