Battery light in instrument cluster

This has a new auxiliary battery, I have tested the auxiliary battery rely power in and out, activated the relay by hand and retested the power in and out, which proved the relay works perfectly, the battery control module does not ground the relay to activate it. I tested the powers and grounds at the battery control module in the trunk all powers and grounds tested good, I cleaned the connector with electrical cleaner spray, removed the battery control module and cleaned the mounting surface.

I also activated the relay from the trunk by grounding the wire to the relay and it activated.

We talked to the dealer they never sold a battery control module and a local Mercedes shop said he never seen one go bad.

This has no battery sensor on the negative cable only the battery control module mounted in the trunk 

Thanks for any assistance 

Fred