S class w222 rear heated screen

[steve.link]

My heated rear screen and heated mirrors aren't working. First question is "are these on the same circuit?
I have checked the fuse/relay, and all seems good. Found this (photo attached) in the owner's manual contained in the COMAND system in the car. The car starts fine and everything else works so I can't see how it's a battery issue.
Any help/pointers much appreciated. - Car is a 2014 W222 S500

 

[Blobcat]

What is the battery voltage like? below ~12.4v with engine off and it needs charging/replacing

 

[steve.link]

What is the battery voltage like? below ~12.4v with engine off and it needs charging/replacing

Battery is fine - relatively new

 

[Blobcat]

Battery is fine - relatively new

The warning above relates to low voltage - which is why I asked

 

[steve.link]

Could it be the auxiliary battery? - I assume the car has one but I don't know where its located?