A 2008 S550 Mercedes we were working on was completely dead.

Symptoms

No interior lights were working. Key lock unlock would not work. We fully charged the starting battery but that still didn't fix the problem.

The starting battery is located in the engine bay on the passenger side. 

Solution

The problem, in this case, was the accessory battery that is mounted in the back.

The battery will either need to be charged with the trickle charger if it is relatively new or it needs to be replaced. This is a 12-volt battery that is mounted between the rear back seat and the trunk.

To fix this problem we charged the battery. The battery charger can be connected at the brass pin located near the battery in the engine bay. It will charge the battery behind the rear seat. 

charging post

You will need to remove the long plastic cover and you will see a thin tall brass pin. We connected the positive clamp to this pin and the negative clamp to bare metal on the car. Allow the car to charge for at least 5 hours then try to start it. Dead Mercedes S-Class

If the car is dead again you may have a parasitic draw or an auxiliary battery that doesn't hold a charge. 

Be extermly careful when connecting the positive to the brass pin. The pin is very close to the fender and you may cause a short.