Mattax said

Then youll have to use voltage drop to test.

One quick way is to turn the headlights on with the engine off, key off.
The ammeter should show about 12 amps discharge.

Measure the voltage at the battery positive, then at the alternator bat stud. There's no current in the alternator output wire so we're just using at as an extension of the probe. Any difference we measure will be due to resistance between the battery positive and the main splice.
We can do one better by measuring direction from the battery poitive to the alternator stud. This eliminates ground path losses.

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