The positive battery cable on my 83 305 nonCCC has this plug on it that currently goes nowhere. What is it?
It Ys off the at the battery post from main positive cable, and on the leg that goes to the alternator. It looks like a watertight plug and its relatively short.
Could be to the ECM, which my car doesnt have?

 

I would say Yes. My factory positive cable (1983 Z28 LU5) had the same plug, main power to the ECM. Or at least that is where the wire terminated.

 

This Thread May Help

What connector is this? - Third Generation F-Body Message Boards

 

Yep, thats it. I get why GM would use the same wires and plugs for multiple optioned cars, but it would be nice if they would have just capped off the unused ones so we know.