Center vents shut off during heating
You shouldn't be getting any air flow from the center vents in the heating mode. The damper on the center vents will only open when you request a much cooler temp than the interior or somewhere I found to be around 70 degrees on the temp wheel.
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'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan