I have an 87 Westy GL with power door locks, windows, and mirrors.
The locks and windows have both given me problems. The lock problem was
similar to yours.  In all cases I replaced expensive VW parts first, only
to discover that the problem persisted.  In each case it was the wiring
coming through the door hinge.  The wires tend to fatigue and break inside
their insulation at this point.  I discovered this after replacing about
$250 worth of VW switches and actuators.
     Unfortunately the wiring of the Vanagon is not up to the quality of
the rest of the vehicle.  I always check it first.

                      John Reddick
                      '87 Westy GL