So I started with what I knew needed to come out, and that was the old siren in the engine bay.
The old seat back closing panel was not nicely fitted
This revealed very damaged heat insulation in the cabin side of the engine bay
With the rear shelf removed (undo seat belts, then just 2 screws and the speaker connections) I could remove the OSR panel and feed the wiring through. Luckily, this immediately got rid of those bare red wires in the earlier pic, which I could cut out. I could then identify that the bug white block was the aftermarket alarm loom, so I could also eliminate that bare black wire which looked like it had been trapped in the seat and ripped in two.
Whilst there, I decided to delete the rear wiper. I was in two minds about this, but it didn't work anyway so out it went. I have ordered a proper blank, but when I paint the car I will fit an RS screen.
So at this point I put the battery back in, and made some covers to prevent earthing on the body (the one over the earth so I can disconnect the terminal and then sit the earth lead on the cover, so it cannot create a contact and make it live again)
I then tested the central locking out of curiosity and it worked! So stripping out the crud, and that black lead either made a short (though less likely as no fuses blown) or more likely making the central locking circuit break was all that it took. Result
So then moving onto the steering wheel, I removed the airbag first (after disconnecting the battery for over half an hour). It was loose, which was no surprise. Given the dash lights, it may not have been activated. So then wheel removed and then the plastic bearing which contains the horn ring, you replace the lot with this OE part from a 928
Then mocked up with the boss
Then refitting the cowl revealed that the boss is a bit larger than OE.
A quick search found a thread on Rennlist suggesting a can of Coke wrapped in sandpaper but I had a new roll of course paper in stock.
This took too long, so I used my belt sander then finished it by hand. I had to be careful though.
With clearance all round I could fit the wheel
Oh, and the horn now works. Tick