Well the install is complete
For those that ever consider a sub install, here's how I did it on a Connect with the full Sync/nav/camera setup:
Power:
There are no decent sources in the cabin or behind the dash - All wiring suitable for the current is terminated with spades, not bolted down. So you'd have to splice in, which is not something I like to do if we're dealing with decent amounts of current (20amps +). So I went for the grommet behind the engine. Remote the engine cover (just pull it firmly away) for better access/visibility. You'll see it. Now go to the glovebox. On the underside of the lid externally you will see two plastic pins. Pull these out and you can then remove the glove box/lid. There is also a cover underneith in the footwell. Removing this is not essential, but is easy to do and helps with cable routing. Thats held in place by two fir tree fixings. Looking through where the glovebox was you'll see a loom going through the firewall into the engine bay. This is the grommet you want. Drill/stab a hole into the RIGHT side of it - The harness is directed immediately to the left when it enters the engine bay. Don't want to stab/drill the loom as I almost did! Now attach your cable to a suitable poking implement and push the cable through the hole, in to the engine bay. Its pretty obvious routing the cable to the battery. The cable was routed in the cab under the centre console, over the pedals (cable tied to dashboard structure) then along the door sill to under the drivers seat. Under the plastic trim of the sill you will find a handy earthing point - The instrument cluster is earthed here too, and I saw the dials twitching as the connection was broke/made, so probably worth disconnecting battery prior to this. I didn't need a remote switch on wire as my sub is auto sensing, but if you need a switched live, then you can get one on the fuse board behind the glove box.
Audio:
Unsurprisingly there are no RCA pre-outs on the head unit, so speaker level it has to be. So now we need to get that lovely dashboard apart. First remove the top centre panel of the dash which shades the monitor. To do that, just grip the front of it over the screen and pull towards you, not up as you will break clips. Now, above the screen are two torx screws. Remove them. Now you can pull out the whole panel with buttons, airvents, and screen bezel in one hit. Try to grip on the edges of the air vents. The whole thing is pretty robust so don't be afraid of giving it some effort. As you pull the assembly away there are two looms plugged into it. unplug these and you can remove the whole panel. Now you'll see a silver box - the ACM (Audio control module). Remove the two torx on the top of it and you can then pull the head unit out revealing two connectors on the back. The one you want is on the left - Connector C240A. Pin-out/wire colours listed below:
Front Left + : Pin 22, White
Front Left - : Pin 10, White/Brown
Front Right + : Pin 23, White/Violet
Front Right - : Pin 11, White/Orange
These four are all located at one end of the connector next to a thicker earth wire. Splice into those accordingly (I use heat shrink adhesive lined splices) Then drop your cables down into the drivers footwell routing over the pedals (cable tie!) and along the sill to under the seat.
Wire that lot up to your sub/amp and you're good to go
The difference is amazing. I've used a Vibe Optisound 8 which came with all required wiring and fits perfectly under the seat. I wasn't after boy racer boom boom boom, more for quality and extending the frequency range of the system, which this little thing does perfectly