Mercedes cls 320cdi

Hi people. im the proud owner of a 2006 mercedes cls 320cdi.  (wish i bloody wasn't).

 

Electrical issues need assistance.  What is the black 2 pin connector joined to positive cable on AUX battery terminal (new aux battery 5 days ago. Have nearly 13v engine off at terminals, but nothing at Aux battery relay?? still get convenience controls disabled message on screen, and car wont turn over? where does connector go? what does it do.