The '87 w124's have no passenger side airbags, so access to the ACC vacuum distribution block and the defroster's diaphragm is quite easy, by removing the glove box. Use a vacuum hand pump to diagnose open circuits. (an inexpensive and quite valuable tool to own!)

Elsewhere on this thread you'll see a link to a vacuum schematic and diagnostic steps for every possible setting. Take your time and write down your results.

I found that replacing just the defroster diaphragm and re-routing the center ones to the floor was quite effective in restoring usable function to the ACC system, albeit cheating a little. I capped off a couple of vacuum circuits that I abandoned. You can order the new diaphragms right from the shop forum sponsors - great value! Don't bother with rebuild kits - get new ones.

Removing the entire dash was more than I wanted to get into, so I simply abandoned the broken flap actuators that were difficult to access.

I drove my 1988 W124 wagon happily for another two or three years in this mode before selling it last year at 220K miles. Should have kept it. I loved that car.

Something to appreciate is the clever strategy of the engineers of this ACC system. The default of a total vacuum failure is to route all air to the defrosters - a good bet in cold climates! The default failure of the defroster control flap is to leave it wide open. You can still operate the car with a clear windshield even if your toes are cold, or hot.

Keep that nice car rolling!

Jerry