As seen below this is a classic case of BWC. You can see the wire has turned
black and is extremely brittle. The contactor from within the plug is no longer
a shiny copper but has become blue with corrosion. Any voltage applied to this
"connector" will no longer have a free and easy path. This corrosion will have
travelled.
If you see any signs of this on your battery, charge leads, switch harness etc,
then it's time to strip it all out, inspect and get new items where necessary
!