well,
I don't want to sound smart, but it could as well be the alarm module.
The door contacts are open when the door is closed. Once opened the contact closes a circuit to chassis ground.
The switch therefore does not sound to be the culprit. If bad, it wouldn't make contact to chassis, no alarm trigger.
It is my understanding that the alarm module, senses the door contacts and a certain condition has to exist in order to trigger an alarm. All of this maybe inside the alarm module...
I can't remember where the module is located at, but have a vage memory somewhere, perhaps the underhood relay box.

Good luck!

P.S.
There should be a momentary switch under the hood driver side. If I am not mistaken this is the Alarm Override switch.
The alarm should go off once pushed. If not, then the alarm module has a problem.
I have found serious solder / contact problems in many of the modules used in those vehicle. They were designed, in my opinion, to last perhaps 10 years, since most Mercedes first owners would get rid of the vehicle on or before anyway.
The problem posted, may as well be, again, just a solder / contact problem.....

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert