I just replaced the bulb that illuminates the open/close dial wheel.
The repair took less than 5 minutes. Here is how it worked on my 1991 124
Take a flashlight and look into the vent. You will see four square shaped clips. Two on the top and two on the bottom. These clips are little pieces of spring loaded metal that hold the vent in place. Carefully take a tiny flat head screw driver and pull the top two clips downwards. You only have to move them down a hair or so. At the same time you want to gently pull out on the vent. Next do the bottom clips, which will be pulled upwards. The vent should come out easily. You should be able to see that all four clips are released. Unplug the bulb harness, remove the bulb and replace the bulb in the socket. To install the vent, line up the vent and then push it in gently.
I hope that this helps some members. This repair is not critical, but I just like to keep my car in 110% at all times and like to share my repairs so others may do them as well.