This might sound stupid, but is the alternator belt still there?  If it was
broken it might explain it.  Also could be some kind of bad ground situation,
or a broken wire to the exciter (internally broken?), or the voltage
regulator decided to die an untimely death.
All of this stuff is easily testible if you have the Jetta Bentley (nicer
than our van books by far) and a multimeter.  Of course you might pay someone
to do this for you if you are too busy or try to take it back to the place
that rebuilt the alternator and get them to test it under warranty (?)

Hope this helps.
Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242