Battery voltage, switches and cut out

[Ian Williams]

Peter, it sounds like the issue is resistance through poor connection, poor earthing or undersize cable. Think of electricity like water, cables like hose, current (amps) like pressure, and resistance like a restriction in the hose. The bigger the hose (cable) the more easily the electricity can flow, place a restrictor in the hose and less water (electrickery) can flow, not so much of an issue if you only need a trickle but with a big demand the resistance will cause a problem. This is very simplified layman's explanation but you don't need to understand all the science, leave that to those who like to tie you up in technicalities.
Try turning all your lights on, and then with a supplementary cable run a direct earth from the battery to the head and tail lights. this will eliminate any doubt about the earth path, but do both head and tail. Then if still an issue run seperate supplementary feeds direct form the battery to both, head and tail lights, if this fixes it you obviously have a resistance somewhere in the supply side. Then try to narrow it down, try a supplementary feed to the light switch, try simply bypassing the switch, remove the battery isolator etc, etc eventually you should narrow it down. I will reiterate that sound connections, adequately sized cable's and clean switch contacts are vital, and often the cause of problems with 6v wiring.