The truck now starts! We have Noise and Action again!
The wire had corroded and come off the connector to the starter button
circuit. We conclude that there is enough conduction when rain gets in, with
the 12V and the differences of the metals (copper wire soldered (lead and
tin) onto a brass spade terminal connector), to allow the copper to
“oxidise” and turn into dust.
We had already started coating other joints with the plumber’s LSX to
provide a waterproof protection, but this one was an old joint and we had
missed it!