83 300D Alternator Exciter Wire Troubles

[Dr.Livingstone]

Hi everybody, I recently bought another 123 for a song and drove it 4 hours home. Initially it charged just fine, starting and stopping in rapid succession with no apparent issues. About an hour away from home we stopped for food and came back out to a dead battery. A jumpstart got us back on the road and we arrived home shortly before dark, thankfully.

Having experienced charging issues on every other W123 I've ever owned I wasn't too terribly surprised, so I dove in as soon as I could. I don't have a multimeter handy, so a test light was fabricated from a 12v bulb and some crimp connectors. Unplugging the harness from the back of the alternator, I found constant battery power at the 2 red wires and nothing at the blue exciter wire with the key on. My charge indicator on the gauge cluster turns on when I turn the key, but still no power at the connector. I pulled apart the ignition switch and jumped the red/black wire (keyed power) with a constant power also in the switch and listened for relays to click and found it to be working.

I took the car to O'Reilly's on a whim to get the alternator checked and the voltage regulator tested bad, though I would assume this is the result of the exciter not doing its job. My next order of business is to trace and visually inspect the wire (as I am unable to actually test continuity), but short of finding a break in the wire I'm kind of at a loss. Is there anything I'm missing or misunderstanding about the charging system? Any help or suggestions would be much appreciated!