Disconnect the jumper wire between the battery cable 'red box' and the alternator output stud, but leave the wire between the alternator output stud and the firewall junction block connected. Reconnect the battery cable to battery. With the ignition switched OFF (i.e., engine not running), probe the alternator output stud with an incandescent test light that is clipped to a shiny clean ground. Does the test light illuminate?
I did this test. After removing the jumper, reconnecting battery, turning ignition to “on” but not engine running the test light does illuminate when touching he output stud with the test light. With the switch in the off position, (key not in ignition) the light remains off. Let me know what I should try next.
You added an extra step by probing the alternator post with the ignition ON...
Repeat the test exactly as you did before, including your extra step, but with
both wires disconnected from the alternator output post.