Most likely it's a bad vacuum element "pod" for the footwell vent flaps. If it's like my 94 e320, you remove the radio to access it. Some models have two. Check the vacuum feed from the controller [behind glove box] and also if the element itself itself holds vac. MB defaults to defrost & side vents only. You should still get AC through center vents.

Bob
$38 MB dealer price, slightly less from online sources.