I'm by no means an electrical expert, but I'll have a go.
I wouldn't be too worried about a short in the harness, i think it would more likley be voltage drop in the whole system as the lights are turned on. Gauges are funny things, and depending on how they work in relation to their sensors, different voltages may effect their readings. I've owned two early commodores, and the blinkers on both cars made the gauges tick in time to the flasher (even worse if the lights were on). Disconnect the lights at the globes, turn the switch on and if it still happens, then it may be a short (I think!!) But, like I said, I'm no expert on things that spark.