the plug in the alternator will have a wire for the warning light and to excite the alternator to charge. with the plug off the voltage will drop because its not actually charging and just reading battery voltage. i mostly work on heavy mining gear and my alternator hasnt had a drama so im not exactly sure how its wired up but i do remember seeing a slightly heavy white wire going through that plug plus another smaller wire, maybe unplug it without engine going and see if voltage at the terminals matches the batt voltage. my first reaction would be to say replace alternator but as you have done that already its unlikely. just one more thing is to make sure the multimeter your using is ok. compare charging rate on a known ok car compared to yours. if you have to use it in the meantime drive with headlighs etc on to help prevent cutting out