Battery Drained

Sounds like you have a short. Charge the battery, put a volt meter between the + battery terminal and cable.

Turn everything off. if you are draining the battery, the volt meter will show currrent flow.

Now start un-hooking stuff until you find the offending device/circuit. Fix short.

offending device...
That's all the electronic cr4p they put on modern cars.
Guy over the road from me has a Merc', he spends a fair bit of time away and has to leave a charger connected to the cigar lighter through a gap in the window (which is then taped to stop any rain gettin in) That's sophisticated German engineering for you.
If they design cars that need trickle charging when not in use, you'd think a discreet socket under the grille would be a neat idea.
And before someone has a pop at me... yes I do know that batteries will self discharge.
Del <click , grumpy mode OFF>

Agreed 100%. There's too much fluff that's electronic/electrical on these new cars.

My newest vehicle is 10 years old and I can't work on it.

But, they do really stupid stuff, too. Like placing the oil filter in such a position that the wheels must be turned sharply to the right to get the oil filter out from under the frame. I discovered this the first time, after I had removed the filter.