I just experienced the problem of the key switch going out on the drivers door - in my case it arms the alarm fine but the microswitch seems to be iffy in the disarm direction with about a 50% success rate. When it goes off go around to the passenger door and turn key to shut off.

In looking around most people just disable the alarm rather than pay the exorbitant $140-180 for a wired microswitch

Anyway since I have a parts car I checked both switches from there - the switches are identical and will fit on either side. However Mercedes in their infinite wisdom wired the pins opposite. So since the drivers door generally gets 99% of the use you can swap the switches provided you cut the harness and swap the two appropriate lines.

This also opens up the possibility of getting a passenger assembly from a junkyard and just use that switch (again you will need to modify the harness.

The connector is circular 3 pin with a flat - pins arranged in a triangle two on the flat one centered above (as you look at the pins). The top pin is the common pin the bottom two swap between driver and passenger door.

Figured this may help
Paul