It's a little hard to comment on a car I'm not familiar with. Generally, you need to have sufficient information to know what is inside the plug-in module you are looking at. My car has a lot of these modules that are relays - they tend to have six pins on the bottom. The only module I have with just two pins is the one with a diode inside it.
The module is mostly plastic, with metal pins of course. The plastic shell has fingers that can be carefully pried apart to allow the shell to come open to reveal the contents. Inside there should be a small diode soldered between the two metal pins.
In my case, the diode was physically broken. The diode component itself is encased in glass or ceramic (or something like that) that is breakable under bending stress. I suspect if the module is bad, physical stress has a good chance of being the cause.
Diodes are one-way devices. They must be properly connected in the right orientation. This is usually marked with a stripe on one end of the diode. Should you decide to have a replacement diode soldered in, this direction orientation must be followed. That is, the stripe end of the diode must always be soldered to the same pin of the module.
It is possible to electrically test the diode module without disassembly. With an ohm meter, you should get low resistance in one direction and high resistance in the other. Try using a resistance scale in the mid-range of what the meter offers. Some meters have a special diode junction testing position.
Too much detail?
Yes, I know. This is one of those tasks where the Mercedes electrical troubleshooting manual was essential.