Electrical Questions / Wiring

It looks like your question expanded to "everything about my car". Always tell us what you have. From the photos, I infer a 1971+ A-body w/ small block, manual tranny, and drum-drum brakes.

With all speakers at max volume (about to smoke), I add up 500W, which would be 42A. Even if you allow 50% loss in the amp, that is only 84 A, which a square-back alternator could manage. Also, you won't always be at max power, unless playing a repetitive rap bass line. Grunge music always has a slow acoustic part where the alternator would be recharging the battery for those "now the part where we play really loud" sections. I think people get carried away in thinking what alternator they need. If your battery stays charged, it is doing its job.