Earthing point rear quarter.

[Howzat]

No wonder you're scratching your head. Try a wire brush on that thread then a washer on first then the wires then nut.

 

[ginkster]

So the brown wire is the original wire that I assume is an earthing wire, the second it the wire that I want to run to a bus bar.

I have the same brown wire which I believe (like you) is from the rear light. All is working fine for me using the stud for my leisure electrics (plus the brown wire attached). As mentioned maybe worth giving it a good clean up to see if that helps. Otherwise no idea what the issue is!! Good luck.