I'm happy to report a successful fix.

With the advice of Zulfiqar, I was able to test my alt to make sure it was good, then do some better research which led me to this thread: 89 300e charging sytem and dash lights - Benzworld.org - Mercedes-Benz Discussion Forum

Turns out the wire from the 2nd terminal of my alt to the x4/10 junction had failed. A quick replacement and everything is working as it should.

The hardest part of the fix was printing the wiring diagram and then figuring out where the junction box was.

Cheers,