i think a little dimming of the headlights at idle is expected.
have you checked voltage at the battery?
battery voltage should be |removethispart|@ 12.5V, higher at idle(13+), and mabe 14.2V- 14.6V when turning faster.
batteries are cheap, and if your alternator is doing this then you should be good.
of course good wiring only helps.........
mabe