I have one on my 90 w124 e300d here in the UK,it is a lock out for the auto box shifter.

There are two lockouts on the auto box shifter unit,one is a cable that upon turning the key in the ignition gets pushed and this allows the shifter to operate.
The other is the switch you have and from 1990 in the UK you have to touch the brake pedal to enable the auto box to be taken out of park.

I replaced my ignition with one from an 89 e300d w124 and this did not have the switch fitted ,but the mounting was there for it,it also did not have the threaded mount for the cable to fit into (it had not been drilled and tapped out) so I fitted a nut onto the threaded end of the cable that would otherwise go into the ignition and then fitted a bolt into this nut and screwed it up until the shifter could be released.

Our w124,s in the Uk do not buzz when ignition is put on,all we get that makes a noise is the lights on alarm which screeches like an air raid siren and this will only sound when a door is opened.