On the verge of giving up entirely.
I took the door card off yesterday (without damaging anything!) with the intention of checking the loom today. After taking the card off, I turned on the ignition and the speakers came on. Great, I thought, must have jiggled something. Then I discovered that, with that speaker disconnected, all the rest work. In addition, SWMBO just told me that she moved the car yesterday a bit to allow builders to park outside the next door neighbour's house - before I had even taken the door card off - and the radio came on full blast for the first time in a week!
That means, logically, that the existing loom is OK and the fault must lie elsewhere.
It's wet and windy and I'm thoroughly pee'd off with it. Just going to clear up and put the door card back. I've officially got the hump with the bloody car.