I felt the need to comment that this is indeed the most annoying key in ignition, seatbelts not latched buzz I have ever encountered on any car I have ever owned.
I would like to make the same 'repair', though I do appreciate the "your lights are on, stupid" warning very much.
Perhaps there is a way to just disable the switch on the ignition cylinder end of this? Sounds like it may be easier than messing with that box, though I am not sure if this switch is integrated with the security system in any way.
On the seatbelt end, I would wonder if the airbags use the same switch to make 'deployment decisions'. Probably not on my 1986 model, but by 1994 this may have evolved.
If anyone knows, that would be great. Those wiring diagrams are never fun to trace...
1986 300E 5-Speed 240k mi.