I really couldn't figure this one out yet, but I used to get a "low battery voltage" light all the time when I'm at a stop at night. It was determined that the battery had gone bad because the the alternator wasn't recharging. So, I replaced the battery and an alternator.

But somehow, this problem seems to continue when I have a few accessories on at the same time (i.e., headlight, defogger, fan), and when I'm at a stop. The last time I was under the car I noticed some engine oil in the alternator main power harness connector. I wonder if this is what is causing the alternator not to recharge fully. Would an oil soak connector caused this problem? When it warms up, I'm gonna try to clean that up and see if it will solve the problem.

Any ideas?

yes its possible ,oil soaked connector could be shorting out and giving bad results/problems/not charging correctly. Cant hurt to dry it out/ maybe even replace the connector

be sure your battery terminals are tight and clean. Also make sure the alternator power wire is tight. Make sure the ground connection at the battery, the one that connects to the fender panel by the battery, is tight and clean.