Battery drain / identify wire near alternator

Short Version: What's this black wire that was clamped onto the alternator, and could disconnecting it keep the battery from charging?

Long Version: One day, when I went to start my car, it was completely dead. It took a jump, but the battery wouldn't hold a charge (except surface charge).

I had Autozone test the battery, and it tested bad so I replaced it. The problem persisted. I had Autozone test the alternator, and it tested bad (putting out 30 amps when it should be 60 amps). I replaced the alternator (with a certified rebuilt), but the problem persists.

Now I understand there could be a drain somewhere in the system. Unfortunately, I don't have a voltmeter to test this, but if it is a drain I probably can't fix it anyway. I'm a complete amateur, and replacing the alternator was by far the most complicated piece of maintenance I've done. However, there's one wire I can't identify, and it seems like this could be causing my problem.

When I took the alternator out, there was a thin black wire that was attached *mechanically* (not in electrical contact) to the alternator. When I pulled on the alternator the wire came with it. Right now one end of this black wire is physically attached to the metal of the car, another goes into a wire bundle along with wires coming out of the alternator, and a third part (there's a "T" junction, so the black wire has three ends) looks like it plugs into something, but is not plugged into anything now.

Is it possible that this little wire should be plugged into something, and it's keeping my battery from charging? If so, how do I find where it should plug in? A mechanic I talked to on the phone said this was probably a ground wire, so all I needed to do was connect it to the metal of the car and the battery would charge, does this seem possible?