Two discoveries, What are they.

[Titus A Duxass]

It's the VW split charge relay. The battery sits in the same compartment.

[ZsZ]

I think the battery connector should be connected to the free pin of the relay but cannot find a picture of the full wiring.

E D I T:

I found the wiring diagram of the original westfalia dual battery setup, I made some comments on it. The gray block called J7 is the split charge relay.
You dont have the fridge and the built in battery charger, so as the wires which are connectig them to the system. But you should have the wire to the second battery positive. That is missing as seen on your picture.


Description how the system works:
If the alternator charges there will be a +12V read on the blue wire,
and if the starter is not engaged, then there will be a ground on the red/black striped wire,
then the relay will switch the two thick red wires together connecting the two batteries.