bop11

The fact that you get a dash light connecting the I terminal to ground but not with the switch on with out the engine running means that there is a problem in the regulator or that the regulator itself is not grounded. There should be a black wire from the headlight harness that connects to the screw holding the regulator to the radiator panel. I assume your headlights work and turn signals work. This black wire is grounded elsewhere at a point that also has a black wire ground from the alternator.
First make sure that the black wire is properly connected to the base of the regulator and there is no paint under it. If so, check that the alternator housing and ground terminal have 0 resistance to the case of the regulator, no broken wires.
If that is ok, then use the above diagram and an ohm meter to insure that the resistors and coils in the regulator are ok.
If that checks out and you still have a problem, then either the wire to terminal S is broken or internal to the alternator the connection from the center of the stator to the STA terminal is broken. Please make sure that the terminal from the S regulator connection actually goes to the STA terminal on the alternator. They are sometimes hard to read under oil or grease.
The ammeter version does not use the STA connection, so the voltage test you did will not test for the stator center to the STA terminal inside the alternator. The regulator S terminal is connected to the ignition switch in the ammeter version and the I terminal is not connected.