I found the same thing today in one of my SD's. The flap for the right footwell opens fine, but the left just swings free. I did have to replace one of the diaphrams. It was the one for the footwell flaps too. It's the pod above and to the left of the glove box. Apparently, it controls the flaps for the left and right feet. Each flap is connected to a control rod via an arm where it exits the heater box.

I need to get a closer look at the that arm. I'm hoping that it is not part of the flap, but attached to the flap in a way that I can reattach it.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)