regarding a 2009 s350.
its a section from the negative terminal lead that goes to the battery. i think the wires have rusted through and its snapped off so just the connector is left.
The auxiliary battery hasn't worked in the car for some time so wondering if this could've been the cause, but right now just need to know if this part (A0065400281) goes by another name because i cant find out online and even the Mercedes dealership doesn't know what the replacement part would be.

A0065400281

here's where the wires have snapped off. one went to the negative terminator and one went to the positive terminal

 

Start with the dealer's diagram of those parts. From the diagram, you can see if it is part of another component. My advice is NEVER to mess with repairs on those cables. If they are bad, feed them to the trash bin and install new OEM cables.