How do I fix the charging system light?

I’ve had a charging system light on my car since I bought it I thought it was a easy fix but it hasn’t and this has caused other issues with my car. I’ve replaced the whole wiring harness for the charging system. The alternator was said to be replaced when I bought it an it looked really new. I’ve tested the voltage on my battery and alternator and it’s within specs. At this point im stuck. The only thing I haven’t looked at is the starter but the vehicle starts every time without a hitch. Maybe it’s not grounding properly? I’m not an electrical specialist on cars so I don’t have a clue.