I just replaced my alternator in my 94 180sx (sr20) and I've noticed that there was a plug that wasn't plugged in. It appears to be apart of the wire harness with the positive and ground leads on the alternator. It's a small one pin plug so I'm assuming it's a ground for something?

I looked everywhere but I didn't see anything that wasn't connected.

Most of the old stuff had a dedicated ground terminal on the alternator with fairly small gauge wire, but the alternator will ground through the casing and doesn't necessarily need the extra contact. People often just removed them. Ohm the connector to ground, if it's connected to ground at the other end then that's what it was.