'86 Westy, manual tran.

My blue wire to the aux battery is not producing 12 volts when the
van is running.  The blue wire from the alternator is producing 12
volts and the starting battery is charging just fine.  I looked at
the wiring diagram in the bently, but can't really figure out where I
should be looking.

Can anybody give me some tips on this?

Thanks
Gregor