Hi Luke, my brother's 495 had a similar problem but IIRC the light would go out when key was off. 884 and 495 should be similar except alternator on his was a Lucas. The problem was internal short in alternator. Replaced with Delco. But you have already swapped alternators. Does light go out when you remove wire from alternator? Usually very simple circuit see attached sample diagram.
Could be short in wiring harness?