found an errant vacuum(black) line....
 

it appears to originate from the three way connector at the vacuum control valve.what is a "vent line to passenger compartment" and should it be loose under and behind the gauge cluster?I discovered this line while trying to trace my center vent issue.BTW the back "pot" that controls center vent and foot vent? seems to be working.any suggestions on where else to look.thanks for any assistance given....

Are you talking about the '84 300D listed in your profile?

You might want to add that to your sig so people will always know what vehicle you're talking about.

I'm not sure about your car, but on mine I believe it's two different pods that control the center vents and the foot vent.

Its a vent line for the vacuum control valve and EGR valve. Leave it alone. It's not supposed to be connected to anything. It is routed into the cabin because the air is cleaner there than in the engine compartment. (Unless the vehicle occupants smoke, in which case the air in the engine compartment is probably cleaner.)

can you expand..does that mean not to worry...or damn! gotta figure out where this goes!

sorry about the omission,figured unless otherwise stated it would be car in my profile.sorry.
those two pots...when lying on my back in drivers foot with head on gas pedal well above my left shoulder is a pot with red cap down and another back along firewall(low) with red cap facing footwell.the second seems to control foot vents and seems to be working fine.is the other pot that I mentioned the one for center vents?when car is running and I manually activate "pot" nothing seems to happen.might be a noise,not surehard to tell.
does this help id where i am working and what I am seeing.

thank you VERY much-one less headache to source!

That sounds like the defrost pod. That's what it is on my car. I suspect you would have to remove the dash to get to the center vents pod...but you're car may be different from mine in that respect.

I have a leak in my center vent pod. It doesn't effect much when it's warm so I've ignored it, but sooner or later I will stick a golf tee in the line leading to that pod from the CCU to stop the leak since I'm not going to remove the dash to do it. Hopefully I can figure out a way to leave the vents open instead of closed.

I found a similar line in my car, I traced it to the diaphram for the cruise control....it had a hole in the line about an inch from the end, just behind the cluster...another "vent line"?