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#1 is as Mike says. #2 is the sense wire. You can sense from the alt's battery post or battery.
#1 is an output wire btw and is tied to the diode trio. Any load greater than an a couple of amps will quickly kill it, and the alternator will go off line.
On modern late model cars with conventional ignition systems (50/60's cars) #1 tied to the key without a diode, light or resistor in line will power the cars coil. It will not turn off with the key.
If you choose to use a wire at #1 you could use a small diode. You also could use the light as it is an indicator that the alternator is working. As it is an output post it turns the light out by suppling the same voltage on the filament as the supply side does. I'd just use a one wire alternator.