Nigel wrote:

I spent yesterday poking about and generally trying to work out how it's
wired, and I found what I think is a relay linking the switched positives.

Well, it has three fat wires going to it, and some thin ones.
You seem to be saying two of the fat ones go to the switched
positives. Where do the others go? If it's a relay, what
energises its coil? Does it go to a switch of some sort, so that
*you* control the relay? Perhaps it's *already* voltage sensitive
and disconnects the alternator from the batteries unless the
alternator voltage is high enough.

One thing that strikes me as odd about this relay is that it looks
like its coil has unusually fat wire. This makes me suspect it's
not a "normal" relay at all, but some kind of "diode" like the ones
that regulators have in them, in other words something that switches
off if current is flowing the wrong way. Is there perhaps (can't
tell from the photo) a thinner winding underneath the thick one?

By the way, what exactly is it you wish your new VSR to do?
Which voltage do you want it to monitor, and when do you want
it to switch on and when off?