An example looks something like this

(It shows an one adapter in the first image but it shows an actual harness further down)

I didn't see one for Mercedes on that site, but it sounds like it mentions the existence of them here under method 3

Installation usually involves:

• Accessing the wiring behind your start/stop button

• Installing the module inline with the existing harness

• Securing the module in a hidden location

That page also suggests that you may be able to disable it with a programming tool on some models. Depending on what you have I'd give that a try first