Where is the auxiliary battery located in my 2011 MBZ E350 4Matic Wagon.

The auxiliary battery is located on the left side of the instrument panel to the left of the steering wheel -- the plastic panel of the instrument panel seen when opening the driver's door. Now, there's nothing saying this needs replaced -- just that there is a malfunction. Testing should be performed. If you replace it, you may still have a problem -- but I understand you may wish to make that educated guess.