Help with wiring please!

[Isaiah92260]

So I have a brand new 120 volt alternator that works great and charges at 14.7, no problems.... But my question is, why does my instrument lights at night dim when I press on the brakes when I'm driving at night? Is that because I have the original wire harness present and I have weak connections? All my alternator wiring I did replace under the hood, I do also have a some what weak batter that gives me 12.3 when the truck is off. I do plan I on eventually replacing my batter and wiring new, just wondering if there is anything I should worry about?