There are two batteries on the CLS500. the main battery is located in the trunk on the passenger side under the trunk mat. There is also a small 12 AH auxillary battery located under the hood beneath the cabin air filter housing, passenger side. This aux battery maintains the electrical system in case the main battery is discharged. It will not start the car.
There is a lighter plug located in the trunk on the driver side , do you know if the source of this plug is coming from the 12 AH auxilary battery or from the main battery that is located in the trunk.