This sounds like the alternator's field wire. Some alternators are 'one wire' in that they are self-exciting with just one big-gauge wire running to the solenoid, battery junction, or some other common hot spot. Alternators with a field wire require a small voltage input from some other source, like the + side of the coil that is only energized when the ignition is on, to 'tickle' or excite the coils in the alternator to start crankin' out the amps.

This is why an engine with a well-and-truly flat battery can't be started by 'popping the clutch' (on cars) or hand-cranking on the A-4. However, if you attach a 6 volt lantern battery to the field wire, you can 'tickle' the alternator into doin' its job: making electricity.

So I would think that this is a diode, in essence, a one-way valve that would prevent current flow back to the coil. You should be able to determine this with a VOM meter....

Cheers