that is the Black box of ******, refering to the box containing all the relays and it sits around the battery. When they get old they oxidize, get brittle, snap off etc. So remove them carefully one by one and clean up with emery paper, may be worth popping the top off to make sure they are clean internally as there can be build up of crud insie too.
When putting the relay back it is crucial to tighten up thespade connectors a bit, but also to ensure that one does not pop out underneath as the relay is being pushed in, it happens alot because the wires get brittle.
Seen as you are in a much warmer country than UK, may not be so crucial, and salt on the roads makes it worse, which we use alot here.