🤔 problem sorted. Turns out the problem is a result of broken plastic tabs in the steering wheel horn button mechanism. There are 4 bolts that are insulated with plastic inserts that isolate both parts of the horn button that make contact when you push the pad. Under 2 of the bolts the plastic insulation was gone which meant both sides of the horn button were making permanent contact. This didn't come to light until the spiral cable was fixed. Toyota claim it was working properly when in left their work shop. If it was, it was a fluke. Anyway it was an easy fix. I made 2 plastic insulating washers to go under the bolts and now all working properly. Toyota sparky didn't diagnose it, I figured it out when I got back from the dealership.