I found another job which would have been marginally easier with the dash out - Botang's hole. Marginally, since the cretin who previously worked on this one had simply cut out the sound deadening covering up the ECU box inner cover and thrown it away, so in fact it was pretty easy to get to the screws with the dash in.
I had to remove the lid of the ECU box with a crowbar
The cretin had siliconed everything shut, and also shattered the bottom of the box at some part and bodged it back together with silicone and bits of cardboard.
This is what the rain tray looked like with the box removed. Clearly it had been messed with multiple times to attempt to stop it leaking. Its a surprise that not once, apparently, in 20 years did it occur to anyone to clean the detritus out of where the rain was collecting.
Bear in mind this is was not found rotting in a field under a tree - this was in daily use until the F125 failed. The D2s I've found in fields have generally been in better condition.
Botang's hole revealing itself with a tiny bit of peeling tape...
Cleaned up a bit. The tape was very dirty underneath and just crumbled away as usual.
A plastic rivet in the hole, covered up with seam sealer. I painted it black later as I don't have any merlin purple paint.
All this original AV harness goes in the bin
I'm just keeping the main body connector for the head unit (switched ignition, illumination, DIN connector, etc), and the CD changer cable for general purpose signalling, as it has 10 wires inside, two of which are shielded. This will get used for a subwoofer level control, possible reverse camera etc.
I might also run a piece of cat5 twisted pair while I'm in there. RCA cables and speaker cables will be run along the original av harness route which carefully avoids as much of the body harness as possible.