The GM internal regulator has two wires in addition to the large output lead. The smaller of the wires on the plug connects through a light (which serves as both a resistance and as a indicator) and serves only as a signal to the alternator to turn on.
The larger plug wire is connected to unswitched 12V and serves as a source of power for the field when alternator RPM is low. An internal shunt power source within the regulator provides field power when RPM is high, which allows the unit to generate even if the field source wire is not connected.
Not enough power can pass through the dash light to actually power the field, so the regulator is designed to separate the signal function and the source function into two different wires. Both are needed for correct operation.
It would be possible to bootstrap the field source wire to the alternator output, but GM has never done this; I suspect that a few feet of wiring is needed to filter out the switching noise of the rectifier set to preclude the regulator going nuts. The field source lead can be connected to a switched 12V source, but is not neccessary as it is blocked by diodes in the regulator.
