Exterior/Dash lights flicker when car is on

So I am having this problem where all of my exterior and dashboard lights flicker/dim when the car is on. I tested the alternator and it's putting out a steady 14.5v. The battery tested fine. The belt tension is fine. Happens with or without any loads put on the alternator/battery. I've tried putting a 8ga wire from the negative terminal to the chassis and block to see if it was a ground issue, but that didn't help. What could this be?