Ever since I have had my coupe the dials and headlights have always dimmed when I had the music on loud, or if I used the electric windows. I have tested the alternator and its fine, and I have replaced the battery. Up until now I have had no clue as to the problem however I have a theory.
When I was driving my car yesterday I turned the bass up on the door speakers and all my headlights and dials were dimming when the bass kicked in. I then completely turned the bass off, but turned the bass up on my sub but the headlights and dials didn't dim. What I am thinking is that its the earth lead causing the problem because surely if the problem was the alternator or battery the sub would cause the headlights and dials to dim, but if the problem was the earth lead the sub has its own one in the boot of my car, where as the door speakers and electric windows have to rely on the standard vauxhall earth lead.
Does that make sense?