Back to the reversing beep volume, I could not find a small speaker under the rear parcel shelf but there was 2 wires with a white socket not connected to anything.
Maybe mine turned up missing in the Australian Compliance procedures, as they have to fit baby seat anchor points to the rear parcel shelf here as standard ( go figure ).
So I traced the sound I do get to the passenger fuse box area. I added some extra holes to the 2 already there to let more sound out. I think I have a win.
I took the opportunity to overcome another niggle. The fuse box cover would always wedge itself on the under dash carpet / padding.
For the first time I noticed the 2 nuts near the hinge with a slide capacity in the hinge to make adjustments.
I elongated the holes in the hinge, but that was still insufficient.
Having removed the metal cover I cut a centimeter from the top, filed the edges, re-painted satin black and we have a winner. No binding.