Volvo XC90 Executive Subwoofer Installation Odyssey (with Focal 1.350 Amp)

Adding Factory XC90 Subwoofer with Focal 1.350 Sub Amp. This is because it's not possible to do a Factory Install of an XC90 Subwoofer to the premium audio system on my car 2009-10 Executive Model ... i.e. the M.O.S.T. optical system does not have a software options for the sub install... just ain't possible.
So the work-around is to drive the sub with an aftermarket sub amp, and since there are no line level outputs, we can use the speaker levels inputs to read a signal from the speaker wires..
What I like about the Focal FD1.350 amp is it's small enough to locate next to the battery, and it has a variable phase adjustment dial though actually I only needed to set to 180 out of phase for optimum sound.
Also the amp is about 2x the power rating of the speaker, so I have used on 20A fuse in the line from the battery to sub amp. Also the speakers are 3 ohm, but the amp says it requires a min of 4 ohm in the dual coil config, but I don't think this is an issue since the amp is well powerful enough and would only be damaged if driving a less than 4ohm speaker very hard.... which won't happen.
Some useful links:
List of parts
https://www.volvopartswebstore.com/showAssembly.aspx?ukey_assembly=1180665&ukey_make=865&ukey_model=11985&modelYear=2009&ukey_category=7858&ukey_driveLine=505&searchString=Subwoofer
Wiring Diagram
https://www.matthewsvolvosite.com/downloads/2005_V70_V70R_XC70_XC90.pdf#page12
The Focal Amp I bought
http://www.caraudiodirect.co.uk/focal-fd1-350-ultra-compact-mono-amplifier
Factory Instructions for Sub install (help with panel removal etc)
http://accessories.volvocars.com/en-se/XC90/InstallationInstruction/Pdf/VCC-138791-1/2006/2.5T%20AWD/Automatic/default/XC90%202006

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