The best way to remove the dash is to have someone do it
You don't have to remove the dash to get to the center vent actuator. Try this:
- open the ashtray
- remove the ash receptacle
- remove the two screws at the bottom of the ashtray frame (don't let them fall under the console)
- pull out the ashtray frame
- reach behind and pull out the radio
- remove the two screws holding the ACC control panel in place
- pull out the ACC control panel
- remove the various electrical connectors and fiber optic lines from the switches. It's pretty hard to get the connections wrong but make labels if it'll make you feel better. No need to label the fiber optic lines, they're all the same.
- look for the center vent actuator in the upper right corner of the cavity. It's casing it pink with a vacuum nipple pointing out at you.
Apply a vacuum to see if it works. If not, scratch your head to figure out how to replace the actuator. I can't figure out how to remove it with or without the dash in place because the far end of the actuator is deep in the vent. The manual says to go in through the venetian blind, but it doesn't lead to the same place. BUT you can get a replacement diaphragm and fix the actuator in situ. Takes some delicate fingers but I was able to do it.
There's a battery of tests you can do before tearing into the console. Start by checking if the system will hold a vacuum downstream of the yellow check valve ahead of the brake booster. You can also check each actuator from the vacuum junction block inside the right console panel. You have to drop the passenger side lower dash panel before you remove the right console panel. Check that each line holds a vacuum and that you can hear a flap actuate. It'll be an obvious and familiar sound if you've been in a 126 with working ACC.
If one actuator is shot you can bet that others are shot as well.