Often overlooked is the short ground wire from engine to firewall. make sure the connections on it are nice and clean. I just dropped the resistance on my sons from 4 ohms neg batt to fender to .3 ohms simply by cleaning this connection.All the stuff said is relevant. I'd also like to add that I've seen weird stuff happen from bad grounds, might be at one of the lights, and then that would make a back-feed ground suck power from the system. Sounds weird, but don't rule anything out until it has been verified.