l have an electrical problem with my 1970 Holden Monaro

It sounds like there’s some kind of problem with the way you have the wiring loom hooked up. One wire in the wrong spot can cause all sorts of mayhem, even in an older car like yours with its relatively wimple wiring. Grab a circuit diagram and try to find out which wire is connected to the wrong terminal.

The other possibility is that the actual light switch has failed internally, allowing the wrong wires to contact each other, or causing them to fail to connect altogether. If you can’t figure it out, it should be a fairly simple and quick fix for an auto electrician to sort.