The pre fuse box by the battery has a few fuses on the outside, however INSIDE it has a relay. Does anyone know what that relay controls?

Mercedes do NOT sell this relay separately however i managed to get one from a breakers as mine had gone.

Now the car is doing some strange things. Its been sat for a week.

I started it fine. Turned it off then it was comeatly dead. Even the hazards wouldnt work. Battery had voltage but it showed a big voltage drop at the pre fuse box.

Its now sat on trickle charge and its dead then its fine then its dead then its fine. When its running its fine. Its also sporadically just lighting up the dash even though the key isnt in it or near it!

Im confused as to what that relay actually does as this may shead some light on the situation