So my center vent is working again. It was the "fresh air door control unit" (hereinafter, the "FADCU").
This isn't earth-shattering, but I figured I'd document my process in case it's helpful to future generations. A lot of this is regurgitation of stuff above, and I'm only trying to organize it for trouble-shooting & repair purposes.
The center vent operation relies on at least three components:
- The stepper motor at the passenger's left foot, just inboard of the glove box (#7 in the RealOEM diagram above)
- The center vent slider control in the AC control pod (called the "fresh air door control potentiometer" in the ETM above)
- The FADCU attached to the ventilation ducting on the driver's side, essentially just opposite of the stepper motor (in fact, the FADCU and the stepper motor are connected to the opposite ends of the rotating shaft that passes through the central vent box and that operates the fresh air vent inside that box - the stepper is on the right side of the shaft, the FADCU is on the left side) (#9 in the RealOEM diagram above)
Based on the failure modes described above, if you're not getting central vent air, or if you're getting it but you can't turn it off because your central slider isn't able to close the vent, then do this:
1. Check your stepper motor. As noted above, drop the glove box, remove the hard plastic cover that's just inside of the glove box. Common failure modes are: (i) screws are loose, (ii) motor has failed, (iii) armature coming off the motor has fallen off or broken.
2. Check your center vent slider control. As noted above, loosen the radio pod and check that the 3-pin connector to the slider is in place.
3. Check your FADCU. This one is tough to access. Remove the kick panel. Look up alongside the ventilation system between the steering column and the ventilation system. There's a box that looks like a tall relay. Under that is a multi-pin connector. Slide them both out of their bracket. You should now be able to see the FADCU. It has a 7-pin socket going into it. The FADCU has a three-link armature connected to the same shaft that connects at the other end to the stepper. Before you remove the FADCU, you will only be able to see two of the arms. In any case, confirm that you see a tall white arm coming up from the FADCU connecting to a shorter white arm that comes down to the vent shaft.
Suggested testing steps:
1. Test the stepper motor. Unplug it from its connector. Apply 12v one way, then the other way. With one polarity, it should rotate 180 degrees one way, and with opposite polarity, it should rotate 180 degrees the other way. Confirm that when the motor rotates, the armature is moving vertically up and down, and in so doing it rotates the shaft that goes into the center ventilation box.
2. Test the slider. Confirm that the 3-pin plug is attached properly. Remove the plug. Attach an ohm-meter between pins 1 and 3. Move slider far left, you should get ~infinity ohms (vent closed). Move slider to the far right, you should get a very low number, something approaching 0 ohms (I think I got 10 ohms)
3. Test the wiring.
- Test continuity of the wires from FADCU to stepper
- Test slider ground (slider pin 2)
- Test FADCU connection to ground at FADCU pin 6
- For the next two tests, make sure the 3-pin connector to the slider is in place
- Turn ignition on, test for 12v+ at pin 5 going into the FADCU
- With the ignition still on, make sure the AC button is off. Confirm 0v at pin 1 into the FADCU. Turn the AC on. Confirm 12v+ at pin 1 into the FADCU.
If your center vent doesn't work but all the tests above are good, then your FADCU is likely bad. They're still available new for $120-ish, but if you can find a part-out I'd guess the part is still there, and probably cheap. In my case, the FADCU armature had been disconnected, and my theory is that since the input shaft wasn't moving the potentiometer in the FADCU, the FADCU just burned out trying to move the stepper.
Stepper motor with armature shown pointing up from the far side of the stepper:
Three-pin connector going into the slider control. From this view, the pins are 1-2-3 reading from left to right
FADCU with armature and socket in place. The longer link coming off the back of the FADCU is obscured in the shadows. The shorter link is visible in the foreground attached to the vent shaft. From this view, the pins are reading 7-6-5-4-3-2-1 from top to bottom. Removing the FADCU, reattaching the armature, and putting the FADCU back in place will be ... challenging.
However, South Florida's weather is very unpredictable as you can see, so opening the doors while driving has proven to be NOT a good option. I could mostly use the E28's fresh ventilation system. Problem is, the bastard doesn't work! I mean, the AC system seems fine as a whole when the blue button is engaged, both Cold/Heat. 
and here in the Forum I believe never read a post regarding how to solve the lack of fresh air flow, but mostly about heater problems. I just want to be able to run fresh air without to resort to any Gullwing doors or other silly and unconventional methods. Cannot afford, need or want either a SLS or any type of convertible. So seriously, can you please tell me what do I have to remove from the ventilation system in my car and if I find it is at fault, have it replaced it in order get fresh air flowing? Hey, hey, no silly electric fan jokes options 
Thanks!
Thank you!
