Given 10c in Suffolk today I got on and finished the job.
These picture from an item on sale on ebay show the section of under dash tray you need to remove, plus there is one screw you can't see at roughly the middle of the glove box 7 another at the far end. There is in total one 10mm bolt & 4 of 3mm allen bolts.
[Linked Image]
[Linked Image]
The under dash tray drops down enough to get the power from the USB socket. All the leads are neatly marked so it is easy to know what is what. I needed to make an adaptor piece as I suspected.

I also could not find the RCA cables behind the dash. I suspect they are lurking by the front air vents and I decided instead to route the set that came with the system under the passenger side carpet as removing the wood over the tunnel I see as quite difficult judging by advice from members on here and the thread on changing the autobox gear lever.

It is all done & dusted now and a road test went well. Last drive of 2024 maybe? xmascheers xmasgrin