There were two different methods posted in the past for how to replace that pesky center vent vacuum pod on a W123, *without* pulling the dash. The first method (from "erubin" at Mercedesshop.com forum) involved removing the center vent eyeballs & changing the whole pod, the second method (from Will Rhyins, on the Ritter list) involved rebuilding the pod in place, leaving the center vents in the car as well. Well I finally tackled the job and ended up using a hybrid of the two methods. I guess you could leave the center vents in there, since Will did that, but it seems a lot easier to work on the pod with them remove. So, consider those steps optional. Here's my edited version, mostly copy & paste from erubin's & Will's posts, with some tweaks based on my install:
First, have a new pod handy (116-800-11-75) and also three of the little attaching metal clips (000-994-96-45). That will cost about $30 or so from Rusty, plus S&H. You may not need the clips but it's worth having them in-hand, just in case, at $1 ea. The "rebuild" kit - actually just a rubber diaphragm - from George Murphy *does* fit this pod, for $5 plus S&H. HOWEVER! The plastic nipple on the pod cover for the vac hose can snap off, since it can be brittle from heat & age. So if you buy the GM kit, you may still be stuck buying a whole new pod if your old one breaks while you're working on it. I had an extra GM diaphragm and used that. You may need to replace some of the Tecalan (plastic) vacuum tubing as well, if it is brittle - hose will work fine if you don't have any. With that out of the way, and assuming you have the parts in your grubby little hands, here's the procedure:
1. Remove glove box completely. I also removed the glove box light switch which got rid of one of the sharper parts digging into my right forearm.
2. Remove both center vents, this is needed so you can work your left hand into the openings. This is where you must have a hand that is small enough to fit into this opening! You will need to remove the rubber duct that attaches to the two center vents. Reach w/ right arm from the glove box opening and hold each vent from the rear. Then turn the front decorative ring about 45 degrees. This should disengage the ring locking tabs from the vent assembly. Pull the decorative rings out the front, then pull the wavy washer rings (through the face of the dash) from the center vent openings. Take note of how the large compression washer assembles (hint they slip in from behind the dash). Pull out the vents from behind the dash (reaching through the glove box). This takes a bit of coaxing, especially for the left vent. I disconnected the electrical wire from the sensor in the middle of the dash, between the 2 vents, because it was in the way.
3. Reach up & across with your hand & carefully remove the rubber vacuum hose atop the center vacuum element. You can barely see it but can easily feel your way. A flashlight stuck in one of the center vent opening helps. The element is attached to a bracket with three round metal press clips. DO NOT REMOVE THIS. If you do, you will have a more difficult job on your hands! Remove only the vacuum hose from the top of the vacuum element.
4. After you've carefully removed the vacuum hose, carefully un-clip the top of the 2-piece vacuum element from the (attached to the bracket) lower piece. Look at the new one you purchased, practice, memorize how it snaps together, do it several times in the dark or without looking before you try to separate & clip together the halves on the one in your car. It's not as hard as you'd think - tools are not required, I could do it with my fingers, at least on the one in the car.
5. With the top half of the vacuum element removed, you can now take out the rubber diaphram. Remove it--NOTE THE POSITION IT WAS IN. Watch out for the big spring inside, don't drop that under the dash.
6. Replace the rubber diaphragm with the one from the new element (or the new diaphragm from George), snap the new cover on. This is not as easy as it sounds. The new diaphragm must be seated perfectly on the cover (pink section). It may be helpful to have a MityVac handy and apply light vacuum to "suck" the new diaphragm in place on the cover before pressing it on to the base in the car. It took me a few tries to make it seal properly.
7. Make sure you seat the rubber element properly, and make sure you snap the vacuum element top correctly to the old base and all should be well. I stuck both hands into the center vent holes. It takes a decent amount of finger pressure to fully snap all 4 tabs into place. Keep the new lower half of the vacuum element as a trophy for your efforts. ;)
8. Test with Mity-Vac before buttoning up to ensure you did it right & that it cured your problem. I did mine at least 2X before I got everything seated properly. Afterwards, mine would leak vacuum slowly, but it worked fine. I didn't attempt to re-do it with zero leakdown.
9. Reinstall the vacuum hose to the element. Start the car, turn the temp wheel to "MIN", and see if the pod operates & opens the flap. Turn it to "MAX, the flap should close. Repeat a couple times to increase your satisfaction of fixing the stupid thing. ;) NOW you are ready to button things back up! (Better to test *before* reassembly, no?)
10. Reinstall the two vents, locking rings and compression washers. This last sentence represented the most difficulty for me. If you removed the electrical wire to the dash temp sensor, put that back too. Put the rubber ducting back onto the vents. I taped the rubber duct on to the left vent which prevented it from becoming dislodged during reassembly. It is a real pain to get both vents in place and the rubber booty back on. Next hassle is getting the decorative trim rings in place. Slip the wavy washer back through the front (don't let it drop!), hold the vent from the rear with your right hand, line up the tabs, and press the front ring on tightly. It should snap into place.
11. Check the foam tube that goes in the pipe to the dash temp sensor. Replace if it's falling apart. Reinstall glove box, light switch (if removed), and all the other assorted stuff you took apart.
12. Drink plenty of wine or beer for the next 5 hours you would normally take removing the dash. Wait two weeks for the knuckle damage to heal. You're done!