I've poked inside the bonnet, and while I don't have an intake in there anymore, or an engine for that matter, I have found three vacuum lines that are possibilities.
The first line goes to the ignition unit and I don't think it is the one because it should be too short to make it to the rear intake. I think I see it in your pic in the very left lower corner.
The second goes through the rear wall next to the fuse box. It connects to a Y where one leg has a check valve and the other leg goes through the firewall.
The third line goes back next to the tranny and connects to a fitting on the side of tranny on the drivers' side. There's a big red circular thing-a-ma-bob where it goes in.
Sorry - I'm not much on tranny speak and I don't have a transmission manual.
'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)