headlight switch

I am changing the lights switch of my Truck..... but Headlighs have their light too weak... When i measure the voltage that they receive, it is only 8.5Volts instead of 12 that have to be... the wire must be grounded somewhere, so i need to change it... but What gauge (thickness) and type of...

Anybody got any suggestions on what is causing my clicking headlight relay to kill my motor? It's on a '62 wagon, 226 with generator a GM system ... I think I have that right. I believe they came with a Ford system, and somewhere along the way my dad changed it to a General Motors system. Or...