Did you try pulling the head lamp fuses in fuse box to kill headlamps? That way you won't need to fool with battery cable.
The voltage to headlamps is hot all the time. Headlamp on/off--hi/lo beam is controlled on the ground circuits. In the diagram, headlamp, the top wire is voltage, the bottom wires are basically grounds, you can see the wire colors. Everything off, Take headlamp connector loose, try a hair dryer or something, make sure plug is dry. Try dielectric grease around outside of connector, I don't know if it will help or not? It's grounding, somewhere? You may have to use test light and check ground circuit. One way to check ground is to use test light touching battery voltage, the other end of test light touches ground wire terminal, if the lite comes on, that circuit is going to ground, somewhere.