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.

If only we had a crystal ball. Unfortunately it's impossible to say with any certainty if the foreign variants of the Monaro will become classics. If any do I would guess it would be the Pontiac with its connections to the GTO, Vauxhall's image in Australia is one of a cardigan wearing, pipe-puffing old timer, and the Chevrolet Lumina doesn't get the pulse racing.

Your complaint is with the mechanic who did the repairs, and you should pursue it through legal means, which appears to be what you are doing.