I contacted Autoleads; here is their rather unhelpful response:

"You will need to find a local car audio specialist who stocks Autoleads products. However I’m afraid that we currently don’t have a PC9 lead that fits your vehicle."

As you can imagine, there aren't any Autoleads stockists in Australia; and, even if there were, they wouldn't have a Sierra loom because the car wasn't sold here.

Back to the drawing board, unless there is kind a PassionForder who is willing try and track one down for me in the UK.