Finding electricity eaters ?

Hello.
I've already used the search but I've found nothing clever!

My problem is my car is just completely empty after 6-7 days!
It doesn't even work the central.

So I'm the only one who has absolutely no idea how to check what it is?

Can anyone tell me how to do that?
Was man dazu braucht ?

I've heard that you should always take out the security separately to test it?