Having some charging issues - ideas?

First, the victim - 1987 Vanagon GL Syncro.

The story - left this morning to go on a 750 mile trip to California.  I
noticed that the voltage displayed by my GPS was low when I started, about
11.9 volts.  Began driving - 11.8, 11.7, 11.6 (you get the idea).  I have
had problems with getting a good voltage reading through the lighter plug
before, at first I put it down to that.  Van was running fine, with
headlights on.  Stopped at a rest area to check the battery voltage with a
voltmeter.  Voltmeter read 12.5 volts with engine off.  Went to start
engine - started to crank then stopped.  Battery voltage now 8.5 volts.
Whoops!!  Not charging.  Got a jump (it started immediately) and turned
around for home (discretion being the better part of valor).

Got ~30 miles to the next rest stop (heading back to Phoenix).  Voltage was
so low the GPS was complaining that it needed to be plugged in (~ 8.3
volts).  Jumped it again, started it and let it run while the other vehicle
continued charging the battery (trying to get a max charge).  Left rest area
for next town (6 miles).  Battery voltage below 9 volts, turned off GPS.
Exited freeway looking for repair shop, found it, left van running, no one
in sight.  Went back to van, cooling fan cycled - I figure battery is really
dead now, after that load.  Turned on GPS, 13.0 volts at idle.  Huh?

Figured the gods of vanagon were smiling on me, I continued for home (I
didn't think they would smile all the way to California).  On the freeway,
voltage kept climbing - to 17.4 volts!  Ouch!  Turned on every electrical
piece I had - headlights, rear window defogger, A/C, fresh air blower, etc.
Voltage dropped to ~ 15 volts.  Fried the battery (well, actually boiled the
battery would be more accurate).

I got a new battery and voltage regulator.  When looking over the
alternator, I noticed that the blue wire which should be connected to D+ was
not connected to anything and taped over.  This wire controls the alternator
light on the dash (which has never worked).  From the Bentley manual, I get
the impression that this is important for the proper working of the charging
circuit.  Could this cause the symptoms above?  I am confused by the not
charging/over charging - past experience says the regulators fail one way -
either they won't charge or they over charge - but they don't flip-flop back
and forth.

Guess I'll try connecting the blue wire and see what happens.  I don't want
to change the regulator if I don't have to, it looks like a pain to do it
with the alternator in place (though Bentley says it can be done, they give
no instructions) and it looks like a pain to remove the alternator (that
lower bolt! - and again, no real instructions in the Bentley manual).

Comments?

Don in Phoenix, AZ
1987 Vanagon GL Syncro
2002 Buell S3T