I had a peek on a Mk2 manual which someone posted a link to a while back.
There only seem to be 2 B/N (bianco/nero = white/black) cables in the whole car, and neither of these could explain anything.
However , the cluster of wires in the earth connector -- (should be something like, several blacks, the dreaded black/white and a black purple) seem to be one of the major secondary earths.
I'd suggest -- humour me -- that you earth that lot out good and proper, and try the original main relay again.