i think your wiring is right trev, the excitor needs 12v to start the charge process, but remember, your feeding into the diode pack, which can cause resistance, it becomes live alternator end when she starts making charge, so not needing exciting any more, and this also puts the light off

but as a rule, you should never run an alternator unconnected, he may have goosed it, i would imagen a diode failure, due to making charge with diodes not excited(this may also be why you have resistance at the excitor)