Hey Anth, did they test the diodes in the alternator? if you have dropped a diode you will lose output.
What sort of lamp do you use? led or normal? if led you must have a resistor inline with the lamp to fake wattage,or check bulb... should be 2 watts (as per bosch?... i have led)
The rb one has a sense wire so will still work with or without lamp but without the sense wire the alternator relies on the lamp to charge.
Your battery may be dead now too! they dont really like any of this ... i went thru 2 listening to experts!
When the last one died i got a big 650cca and ordered the 105amp rb30 one and instant success.
My battery is in the boot with 2gauge to starter, neg is earthed to rear bumper. I also used the red 2gauge from jaycar to go from alternator to starter, see photos previous. With my one before i had to give a rev to get it to start charging and was a pain when cold starting and then died... our thoughts was one of the diodes went... could have been all the jumpstarting and charging the battery on trickle because alternator couldnt keep up.
For the effort you have put in for this frankenstein and auto electricians, pita!
new one much easier... goodluck mate! i hope you get it sorted