Unrelated but similar for what it's worth...
I just had an earth prob (huge cloud of smoke out the dash as I'm driving along) and it was the black wire that runs from the fuse box and earths on the speedo housing. What caused the meltdown was the earth connection to the fuel guage, there is a fibre washer that isolates this connection from the metal housing and with age it had fallen off causing said connection to touch the metal.
When I found this out I noticed that my fuel needle banged over to full every time I made the two touch.