StoneX.....yes i know how headlight dimming is caused, most of the time it can be solved by upgrading your earths under the bonnet, what i should have suggested after that is to get your alternator looked at. Bad headlight dimming usually means the voltage is dropping below 12.x volts (battery voltage) anyway, hence the reason for me suggesting a new/bigger/second battery.
acarmody.....i would run another earth from the neg. terminal to the engine block etc.(alternator housing bolt is ideal), just to make sure your getting as much charge current as possible.