In message , The Natural Philosopher
writes
I suspect its a subspecies of this:

http://www.barbadosrc.org/misc/blackwire.php

which is well known, if not so well understood.
Which doesn't address the problem in cars. At all.

I suspect myself its electrochemical attack of copper due to three
things: damp, a salt, and a voltage.
And **** wire.

No. Not **** wire.

Which is the point.

Umm.. aren't apocryphal tales told of bodywork corrosion when Ford
changed the battery polarity on their cars?

Tinned multistrand cable is OK on my tractors which have -ve earth.

regards

--
Tim Lamb