Oddly enough a couple of years ago a customer asked me to change the
alternator on a golf, the AA or Rac had diagnosed it as faulty, the
customer supplied the alternator from GSF and it was the wrong one,
eventually the right one was supplied and I fitted it, same problem, no
charge and no charge warning light. I checked out the charging light
circuit and found there was no connection from alternator to dash light,
found the problem of a broken wire, somewhere near the battery IIRC and
repaired it. That whole escapade cost the customer about 200 quid more
than it needed to have done.