The alternator's ground is normally done to one of the four long bolts that run through the whole assembly, binding the end caps to the stator core. It is somewhere between an 8 and 10 gauge blue wire leading to one of the bolts mounting the alternator bracket, I think, in order to make up for the fact the unit is electrically isolated from ground by the rubber bushings. I figured you meant "regulator" when you said "rectifier", so I related my story about the symptoms of a missing regulator ground.

The "ground" that turns on the battery light when the alternator is still is made through the regulator and brushes for the rotor. There are 9 diodes inside the alternator, the six high-current diodes that do the main job of rectifying the charging current, and three diodes from the stator windings to provide the rotor current once the alternator is turning. That same voltage is seen by the red wire back to the instrument cluster, but not until the alternator is producing energy. The three small diodes (actually in one package) are facing the wrong direction to provide a ground for the battery light.

How did your test work out-- grounding the small red lead while looking at the cluster idiot lights?
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Art Benstein near Baltimore