[VetteVet]

Do you have a four pin alternator or just the one wire to "L" ? I believe the ECM controls the exciter voltage to "L" but it uses less than than 12 volts and it is switched to ground. You should be able to control the alternator but don't add anything to the "L" or you will probably fry the ECM.

If your ammeter gauge wiring is in place you shouldn't need to connect anything off the alternator for it to work.
You should have 2 wires in the harness with 2 small inline fuses (4 amp) in each wire. One over near the battery and one on the main junction of the alternator output wire and the battery, and cab wires. These two wires, one black and one black with white stripe, run into the cab to the cluster plug to terminals 1 and 12. I have a FAQ thread in the electrical forum with more info on the ammeter also called the battery gauge.

If You have an alternator with the four terminals labeled " P L I S " or " F S L P " Then you should be able to control the alternator without the ECM.
You would need to run a wire from the key switch accessory terminal with an 85 ohm 5 watt resistor inline to the "L" terminal and one from the main junction to the "S" terminal for voltage sensing. There should be several wiring diagrams in the FAQ thread to illustrate what I have said.