Getting closer, started figuring out where I want to mount everything in the trunk, decided on something very similar to this, just with the DSP at the top right in the picture instead of centrally mounted. It makes things a little simpler with managing wires and getting access to the USB on the front of the DSP. Along with that I cut the power and ground wires to length from the distribution blocks and located what I hope should be a decent ground in the trunk. Along with that I ran the high level input from the factory amp to the trunk, essentially followed the speakers from the front doors, wrapped in techflex and some tesa tape.
Not much to look at, the darker green plug will be replaced with the lighter green plug and that blue wire will get spliced into the red wire on the darker green plug. I have no idea why it worked previously with that red wire (or blue in the case of the light green plug) not connected, but apparently it does need to be connected. I did find out that there is a MOST25 adapter available, that will provide an optical input to the DSP, but at the moment I can't really justify the $600 cost of it. Especially as I just had to call an HVAC company to come figure out why the indoor coils keep freezing over, did my own troubleshooting, but no gauges to check the refrigerant level. Anyway, getting off track. Eventually I may pick up the Helix SDMI25 and swap to optical, but for now it's something that will have to wait.
To get a switched 12V I had to tap off the back of the front 12V socket between the cupholders, this meant removing the entire HVAC panel, in order to access a pair of screws just before the shifter to release the center console. I tried using a wire tap to retain use of the 12V socket, but for some reason never have any luck with those, since I rarely ever use the 12V socket I decided to just disconnect it and use spade terminals. At some point I may revisit getting the socket working again, but for now not overly concerned.
Since I had the center console taken apart I took the time to also run the DSP remote and a coax cable for an Fiio from the arm rest to the trunk as well.
I'd prefer a more fixed location for the DSP remote, but so far have not found any places suitable, so it may just stay in the arm rest. If I get time to this week I plan to drill a couple holes in that factory tray to pass power, ground and signal wires through before starting to permanently mount everything to it. With that done the trunk should come together fairly quickly and I can then move onto mounting speakers in the front doors. If everything goes smoothly, hopefully another week or two and I should be up and running.