Electrical 2000 V6 Mustang lights flicker

Hey guys, I've had this issue since January. The problem I'm having is when the car is running, all the lights flicker (Headlights, interior, cluster). I have played around with it and managed to stop the flickering only with the headlights and defrost on. I have cleaned all the engine grounds, replaced battery, alternator, belt, still having that issue.. any suggestions? Thanks.

Cleaned both grounds on the radiator support(Above both headlights), engine-to-chassis(under driver side of engine to driver side chassis), Chassis to hood(Really needed?). The current voltage is between 14.59v - 14.75v with engine idle at both points. I will check out that link and see if it helps.