It's in poor condition but obviously still working. Put some de-oxit on and wrap it for now. Clean up the battery ternimals too. That will all help reduce resistance.
My guess is the big splice is in the R6 circuit, and that its bypassing the engine connector. So again the engine connector is suspect as to why the alternator is not working.
Based on your location, I'd look for marine electrical supplies as a source of moisture resistance splice connectors and wiring.